Add mutation for deleting account

[fowczarek]

I want to merge this change because resolve #4286

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 d849171 (mirumee/saleor @ 4289/Change-logged-user-email-and-names)
Base comparison is abfcf33.

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

[codecov]

Codecov Report

Merging #4494 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4494      +/-   ##
==========================================
+ Coverage   90.65%   90.66%   +<.01%     
==========================================
  Files         294      294              
  Lines       16961    16971      +10     
  Branches     1720     1720              
==========================================
+ Hits        15376    15386      +10     
  Misses       1081     1081              
  Partials      504      504

Impacted Files	Coverage Δ
saleor/graphql/account/mutations.py	97.51% <100%> (+0.05%)	⬆️
saleor/graphql/account/schema.py	100% <100%> (ø)	⬆️

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[NyanKiyoshi]

LGTM