Make change reason customizable with overriding.

[amartis]

Description

Shortcoming of the previous architecture:
Recording the reason for a change was only possible through manually setting to each object or through bulk_history_create.
So recording API request as the reason (of the change) automatically was hard to implement with this architecture.

Solution:
Getting change reason is now done through HistoricalRecords method get_change_reason_for_object.
And thus this method can be overriden in subclass to define custom behavior.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #962 (031ba14) into master (fc9b55e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #962   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files          23       23           
  Lines        1096     1098    +2     
  Branches      213      213           
=======================================
+ Hits         1073     1075    +2     
  Misses         10       10           
  Partials       13       13           

Impacted Files	Coverage Δ
simple_history/models.py	98.36% <100.00%> (+<0.01%)	⬆️

---

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 fc9b55e...031ba14. Read the comment docs.

[amartis]

@jeking3 I made the change in PR #948 onto master branch as you recommended.
Will you review this PR and approve?
FYI, this change is successfully running in a Django project of mine.

Thanks for reviewing previously, btw!