Add ability to store payment source

[salwator]

Adds ability to store credit card information on payment.

Closes #2700

This PR introduces:

• new private meta field for user to store external data, ex. gateway information
• new interface for extending parameters for client token generation, ex. braintree token for existing customer
• new flag in CheckoutComplete mutation to rerquest saving user payment source in gateway (needs to be enabled in gateway settings too)
• new flow for charging existing customer instead of one-time-token each time, implemented in Braintree gateway
• improvement in testing, most sensitive tests in braintree gateway test suite are now gone and replaced with VCR-based containing real life sandbox communication
• new gateway method for retrieving customer stored payment sources (credit cards)
• fixes bug with billing and shipping addresses causing call_gateway to fail

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. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[NyanKiyoshi]

Closes #2700?

[codecov]

Codecov Report

Merging #4195 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4195      +/-   ##
==========================================
+ Coverage   91.77%   91.77%   +<.01%     
==========================================
  Files         278      278              
  Lines       15548    15552       +4     
  Branches     1513     1513              
==========================================
+ Hits        14269    14273       +4     
  Misses        879      879              
  Partials      400      400

Impacted Files	Coverage Δ
saleor/account/models.py	93.7% <100%> (+0.04%)	⬆️
saleor/payment/interface.py	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #4195 into master will increase coverage by 0.01%.
The diff coverage is 93.22%.

@@            Coverage Diff             @@
##           master    #4195      +/-   ##
==========================================
+ Coverage    90.7%   90.72%   +0.01%     
==========================================
  Files         291      291              
  Lines       16680    16792     +112     
  Branches     1691     1708      +17     
==========================================
+ Hits        15130    15234     +104     
- Misses       1054     1058       +4     
- Partials      496      500       +4

Impacted Files	Coverage Δ
saleor/graphql/account/resolvers.py	95% <100%> (+1.66%)	⬆️
saleor/payment/models.py	94.89% <100%> (+0.05%)	⬆️
saleor/graphql/account/types.py	96.22% <100%> (+0.14%)	⬆️
saleor/payment/interface.py	100% <100%> (ø)	⬆️
saleor/graphql/payment/types.py	85.22% <100%> (+0.88%)	⬆️
saleor/graphql/payment/resolvers.py	100% <100%> (ø)	⬆️
saleor/graphql/payment/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/checkout/mutations.py	94.06% <71.42%> (-0.56%)	⬇️
saleor/account/models.py	93.23% <85.71%> (-0.42%)	⬇️
saleor/payment/utils.py	90.47% <86.36%> (-0.61%)	⬇️
... and 8 more

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[django-queries]

Here is the report for ad32191 (mirumee/saleor @ store-credit-card-info)
Base comparison is d528263.

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                     	         43	         43	             21

# 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                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	          9	          9	              2