Implement customer events

[NyanKiyoshi]

Closes #4039.

Checklist

• Documentation
  • Document changes in a sub section "Customer Events"

• Model changes
  • Add a CustomerEvent model and types

• New Customer Events
  • the order was placed
  • a note was added to the order
  • downloaded digital link
  • account was created
  • password reset link sent
  • email assigned
  • note added
  • name added => first_name: str|none; last_name: str|none

• New Staff Events
  • A customer or customers were deleted (event with a count parameter)

• GraphQL Changes

  • A countable object from the CustomerEvent model
  • A events field in customer

• Misc

  • Move incremental to product.utils.digital_download

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.

[codecov]

Codecov Report

Merging #4094 into master will increase coverage by 0.13%.
The diff coverage is 97.96%.

@@            Coverage Diff             @@
##           master    #4094      +/-   ##
==========================================
+ Coverage    91.5%   91.63%   +0.13%     
==========================================
  Files         277      278       +1     
  Lines       15151    15308     +157     
  Branches     1489     1503      +14     
==========================================
+ Hits        13864    14028     +164     
+ Misses        882      879       -3     
+ Partials      405      401       -4

Impacted Files	Coverage Δ
saleor/order/models.py	89.62% <0%> (ø)	⬆️
saleor/account/events.py	100% <100%> (ø)
saleor/dashboard/customer/forms.py	92.64% <100%> (+3.06%)	⬆️
saleor/account/views.py	81.55% <100%> (+0.94%)	⬆️
saleor/account/emails.py	77.77% <100%> (+27.77%)	⬆️
saleor/graphql/account/bulk_mutations.py	100% <100%> (ø)	⬆️
saleor/graphql/account/mutations.py	97.34% <100%> (+1.26%)	⬆️
saleor/order/events.py	100% <100%> (ø)	⬆️
saleor/account/forms.py	88.09% <100%> (+2.72%)	⬆️
saleor/product/utils/digital_products.py	96.77% <100%> (+1.12%)	⬆️
... and 9 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 32edec1...7308a99. Read the comment docs.