Add voucher settings

[benekex2]

Ref #4434

Screenshots

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.

[django-queries]

Here is the report for c25bbae (mirumee/saleor @ add-voucher-settings)
Base comparison is 94314bf.

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                     	         41	         41	             20

# 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

[codecov]

Codecov Report

Merging #4556 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4556      +/-   ##
==========================================
+ Coverage   90.64%   90.66%   +0.01%     
==========================================
  Files         297      297              
  Lines       17411    17411              
  Branches     1738     1738              
==========================================
+ Hits        15783    15785       +2     
+ Misses       1121     1120       -1     
+ Partials      507      506       -1

Impacted Files	Coverage Δ
saleor/payment/utils.py	91.34% <0%> (+0.86%)	⬆️

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[codecov]

Codecov Report

Merging #4556 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4556   +/-   ##
=======================================
  Coverage   90.88%   90.88%           
=======================================
  Files         296      296           
  Lines       17435    17435           
  Branches     1734     1734           
=======================================
  Hits        15845    15845           
  Misses       1083     1083           
  Partials      507      507

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[dominik-zeglen]

Let's extract this to function

[benekex2]

Why? This is used only once.

[dominik-zeglen]

It's the same as
https://github.com/mirumee/saleor/pull/4556/files/9fd99d048ad050ea247a01fef15305aeea6375bb#diff-95dd24f9158deb91b0d37a41f830804cR339
so it can be extracted to helper function at the section level