Allow to delete category and leave products

[IKarbowiak]

Allow to create product without category. After removing category, related products become unpublished and minimal variant price is recalculated.

Closes #1928

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[django-queries]

Here is the report for d79112b (mirumee/saleor @ allow-to-delete-category-and-leave-products)
Base comparison is 5f5936a.

No differences were found. (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         34	         34	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         14	         14	              0
  complete checkout                          	          6	          6	              0
  create checkout                            	         50	         50	             24

# api.benchmark homepage
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve main menu                         	          5	          5	              0
  retrieve product list                      	          4	          4	              0
  retrieve secondary menu                    	          5	          5	              0
  retrieve shop                              	          2	          2	              0

# api.benchmark product
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product details                            	         15	         15	              3
  retrieve product attributes                	         13	         13	              2

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         15	         15	              6

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0

[codecov]

Codecov Report

Merging #4970 into master will increase coverage by 0.01%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master    #4970      +/-   ##
==========================================
+ Coverage   91.17%   91.18%   +0.01%     
==========================================
  Files         351      351              
  Lines       21340    21391      +51     
  Branches     2037     2042       +5     
==========================================
+ Hits        19457    19506      +49     
- Misses       1328     1329       +1     
- Partials      555      556       +1

Impacted Files	Coverage Δ
saleor/dashboard/product/filters.py	100% <ø> (ø)	⬆️
saleor/product/models.py	93.77% <ø> (ø)	⬆️
saleor/dashboard/product/forms.py	92.85% <100%> (+0.15%)	⬆️
saleor/core/models.py	100% <100%> (ø)	⬆️
saleor/graphql/product/bulk_mutations/products.py	96.93% <100%> (+0.04%)	⬆️
saleor/dashboard/category/views.py	83.87% <100%> (+0.17%)	⬆️
saleor/product/utils/__init__.py	95.83% <100%> (+1.02%)	⬆️
saleor/graphql/product/filters.py	92.97% <100%> (+0.11%)	⬆️
saleor/product/tasks.py	100% <100%> (ø)	⬆️
saleor/graphql/product/mutations/products.py	95.32% <86.66%> (-0.16%)	⬇️

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[maarcingebala]

Good job 👍

[NyanKiyoshi]

LGTM 👍