Update default decimal places

[IKarbowiak]

Changing currency caused errors when running Saleor.

• Set DEFAULT_DECIMAL_PLACES to 3, which should be enough for all currencies (https://en.wikipedia.org/wiki/ISO_4217).
• Create mutations for changing decimal_places in DecimalFields
• Update Money ObjectType - return amount with the number of decimal places corresponding to a given currency
• Create PositiveDecimal that contains validation - prevent for creating an item with a negative value
• Add price amount validation - check if the price amount does not have more decimal places than the given currency allows

⚠️
Require frontend changes

Resolves #5360

Impact

• New migrations
• New/Updated API fields or mutations
• Deprecated API fields or mutations
• Removed API types, fields, or mutations
• Documentation needs to be updated

Pull Request Checklist

• Privileged queries and mutations are guarded by proper permission checks
• Database queries are optimized and the number of queries is constant
• Database migration files are up to date
• The changes are tested
• GraphQL schema and type definitions are up to date
• Changes are mentioned in the changelog

[codecov]

Codecov Report

Merging #6098 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6098      +/-   ##
==========================================
+ Coverage   91.78%   91.80%   +0.02%     
==========================================
  Files         389      389              
  Lines       25554    25621      +67     
  Branches     2410     2407       -3     
==========================================
+ Hits        23454    23521      +67     
- Misses       1515     1516       +1     
+ Partials      585      584       -1     

Impacted Files	Coverage Δ
saleor/tests/fixtures.py	95.28% <ø> (ø)
saleor/graphql/core/scalars.py	82.02% <100.00%> (+1.53%)	⬆️
saleor/graphql/core/types/money.py	100.00% <100.00%> (ø)
saleor/graphql/core/validators.py	100.00% <100.00%> (ø)
saleor/graphql/discount/mutations.py	98.61% <100.00%> (+0.03%)	⬆️
saleor/graphql/giftcard/mutations.py	98.92% <100.00%> (+0.07%)	⬆️
saleor/graphql/order/mutations/draft_orders.py	95.29% <100.00%> (ø)
saleor/graphql/order/mutations/orders.py	95.57% <100.00%> (ø)
saleor/graphql/payment/mutations.py	95.48% <100.00%> (ø)
saleor/graphql/product/bulk_mutations/products.py	97.54% <100.00%> (+0.05%)	⬆️
... and 4 more

---

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[db-queries]

Here is the report for 6b5bf47 (mirumee/saleor @ update-default-decimal-places)
Base comparison is efc27eb.

**Found 1 differences!** (click me)

# saleor.graphql.accountbenchmark account
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  delete staff members                                       	         32	         32	              0
  query staff user                                           	         20	         20	              4
  staff create                                               	         22	         22	              4
  staff update groups and permissions                        	         33	         33	              5

# saleor.graphql.accountbenchmark permission group
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  permission group create                                    	         19	         19	              2
  permission group delete                                    	         20	         20	              3
  permission group query                                     	          7	          7	              0
  permission group update                                    	         33	         33	              1
  permission group update remove users with manage staff     	         27	         27	              3

# saleor.graphql.checkoutbenchmark checkout mutations
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  add billing address to checkout                            	         72	         72	             42
  add shipping to checkout                                   	         50	         50	             19
  checkout email update                                      	         41	         41	             21
  checkout payment charge                                    	         38	         38	             10
  checkout shipping address update                           	         58	         58	             19
  checkout voucher code                                      	         85	         85	             53
- complete checkout                                          	        118	        119	             25
  create checkout                                            	        130	        130	             64
  update checkout lines                                      	        114	        114	             54

# saleor.graphql.checkoutbenchmark homepage
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  user checkout details                                      	         51	         51	             27

# saleor.graphql.menubenchmark homepage
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  retrieve main menu                                         	          5	          5	              0
  retrieve secondary menu                                    	          5	          5	              0

# saleor.graphql.orderbenchmark order
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  user order details                                         	         17	         17	              3

# saleor.graphql.productbenchmark category
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  category view                                              	         18	         18	              1

# saleor.graphql.productbenchmark collection
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  collection view                                            	         17	         17	              0

# saleor.graphql.productbenchmark homepage
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  featured products list                                     	         14	         14	              0
  retrieve product list                                      	          5	          5	              0

# saleor.graphql.productbenchmark product
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  product details                                            	         16	         16	              0
  retrieve product attributes                                	          7	          7	              0

# saleor.graphql.productbenchmark variant
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  product variant bulk create                                	         47	         47	              2
  retrieve variant list                                      	         18	         18	              2

# saleor.graphql.productbenchmark variant stocks
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  product variants stocks create                             	         22	         22	              5
  product variants stocks delete                             	         19	         19	              5
  product variants stocks update                             	         27	         27	              5

# saleor.graphql.producttest product sorting attributes
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data                     	         20	         20	              0

# saleor.graphql.shopbenchmark homepage
  test name                                                  	left count 	right count	duplicate count
  -----------------------------------------------------------	-----------	-----------	---------------
  retrieve shop                                              	          2	          2	              0

[maarcingebala]

Looks really good!