Mutation to change user email

[kswiatek92]

closes #4648

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.

[codecov]

Codecov Report

Merging #5076 into master will increase coverage by 0.03%.
The diff coverage is 97.64%.

@@            Coverage Diff             @@
##           master    #5076      +/-   ##
==========================================
+ Coverage   91.53%   91.56%   +0.03%     
==========================================
  Files         260      260              
  Lines       16717    16802      +85     
  Branches     1469     1472       +3     
==========================================
+ Hits        15302    15385      +83     
- Misses       1034     1035       +1     
- Partials      381      382       +1

Impacted Files	Coverage Δ
saleor/account/events.py	100% <100%> (ø)	⬆️
saleor/account/emails.py	89.39% <100%> (+4.94%)	⬆️
saleor/graphql/account/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/account/mutations/account.py	99.15% <96.55%> (-0.85%)	⬇️

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[django-queries]

Here is the report for 8e875bf (kswiatek92/saleor @ mutation-to-change-user-email)
Base comparison is b34b31e.

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                    	         10	         10	              0
  complete checkout                          	          8	          8	              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                            	         14	         14	              2
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         16	         16	              6

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