M2M support for inheritance and signals

[legau]

Some M2M enhancements that were not yet merge into #932

Description

This makes it possible to use the new m2m fields feature with model inheritance.
It also introduces some pre and post m2m save signals.

Motivation and Context

Abstract models could not be used with previous implementation of historical m2m fields.

How Has This Been Tested?

Added relevant tests

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 #1042 (7042a64) into master (7042a64) will not change coverage.
The diff coverage is n/a.

❗ Current head 7042a64 differs from pull request most recent head a8c08b7. Consider uploading reports for the commit a8c08b7 to get more accurate results

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  Coverage   97.11%   97.11%           
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  Files          23       23           
  Lines        1215     1215           
  Branches      237      237           
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  Hits         1180     1180           
  Misses         17       17           
  Partials       18       18           

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