Fix O(n) queries when adding M2M objects

[ddabble]

Description

For history-tracked M2M fields, adding M2M objects (using add() or set()) used to cause a number of database queries that scaled linearly with the number of objects (O(n)); this has been fixed to now be a constant number of queries (O(1)).

Related Issue

Resolves #1317.

Motivation and Context

See linked issue.

How Has This Been Tested?

See the added test.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 96.84%. Comparing base (733f4e0) to head (a089803).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1333      +/-   ##
==========================================
+ Coverage   96.83%   96.84%   +0.01%     
==========================================
  Files          24       24              
  Lines        1452     1457       +5     
  Branches      237      240       +3     
==========================================
+ Hits         1406     1411       +5     
  Misses         24       24              
  Partials       22       22              

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[tim-schilling]

Nicely done!