Send fulfillment update email

[IKarbowiak]

Send fulfillment update mail after updating the tracking number in FulfillmentUpdateTracking mutation

Closes #4017

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 #5118 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5118      +/-   ##
==========================================
+ Coverage      91%   91.13%   +0.12%     
==========================================
  Files         259      261       +2     
  Lines       16751    16860     +109     
  Branches     1489     1502      +13     
==========================================
+ Hits        15245    15365     +120     
+ Misses       1103     1094       -9     
+ Partials      403      401       -2

Impacted Files	Coverage Δ
saleor/graphql/order/mutations/fulfillments.py	97.91% <100%> (+0.07%)	⬆️
saleor/graphql/payment/schema.py	95.83% <0%> (-4.17%)	⬇️
saleor/graphql/checkout/types.py	95.19% <0%> (-0.81%)	⬇️
saleor/graphql/account/mutations/base.py	98.24% <0%> (-0.42%)	⬇️
saleor/graphql/account/mutations/staff.py	96.7% <0%> (-0.33%)	⬇️
saleor/graphql/checkout/mutations.py	94.05% <0%> (-0.11%)	⬇️
saleor/graphql/checkout/schema.py	100% <0%> (ø)	⬆️
saleor/graphql/page/schema.py	100% <0%> (ø)	⬆️
saleor/graphql/order/schema.py	100% <0%> (ø)	⬆️
saleor/graphql/account/schema.py	100% <0%> (ø)	⬆️
... and 14 more

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[django-queries]

Here is the report for eec7116 (mirumee/saleor @ send-fulfillment-update-emails)
Base comparison is 9502467.

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                            	         15	         15	              3
  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                      	         15	         15	              6

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