Remove all static payment settings

[salwator]

This PR purpose is to remove all remaining static configuration for the payment process and rely entirely on payment plugins configuration. This forces us to abandon payment gateway enum from graphql schema and fetch template_path information from plugin configuration.

Fixes #4748

Roadmap:

• Move template_path to the plugin configuration
  • Dummy
  • Braintree
  • Razorpay
• Use plugin template_path in payment django view
• Remove template_path from settings.py
• Remove PaymentGatewayEnum, change to String
• Remove PAYMENT_GATEWAYS from settings
• Remove Gateway enum from payment interface

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 078f3d1 (mirumee/saleor @ remove-all-static-payment-settings)
Base comparison is 7478b7c.

No differences were found. (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         38	         38	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         14	         14	              0
  complete checkout                          	          6	          6	              0
  create checkout                            	         52	         52	             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
  -------------------------------------------	-----------	-----------	---------------
  retrieve variant list                      	         18	         18	              8

# 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 #4756 into master will decrease coverage by 0.04%.
The diff coverage is 56.52%.

@@            Coverage Diff             @@
##           master    #4756      +/-   ##
==========================================
- Coverage   91.15%   91.11%   -0.05%     
==========================================
  Files         343      343              
  Lines       20371    20366       -5     
  Branches     1930     1930              
==========================================
- Hits        18569    18556      -13     
- Misses       1264     1272       +8     
  Partials      538      538

Impacted Files	Coverage Δ
saleor/graphql/account/resolvers.py	100% <ø> (ø)	⬆️
saleor/graphql/checkout/types.py	94.11% <ø> (-0.07%)	⬇️
saleor/graphql/payment/mutations.py	96.39% <ø> (-0.04%)	⬇️
saleor/graphql/payment/schema.py	95.45% <ø> (-0.2%)	⬇️
saleor/payment/gateways/stripe/plugin.py	0% <0%> (ø)	⬆️
saleor/payment/gateways/razorpay/plugin.py	0% <0%> (ø)	⬆️
saleor/payment/gateways/braintree/plugin.py	0% <0%> (ø)	⬆️
saleor/graphql/payment/enums.py	89.47% <100%> (-0.53%)	⬇️
saleor/payment/gateways/dummy/plugin.py	91.93% <100%> (+0.41%)	⬆️
saleor/payment/gateway.py	95.52% <100%> (+0.96%)	⬆️
... and 4 more

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[korycins]

@salwator What about storefront PWA. Are they ready for this change?