Added support for SynchronizedField on ManyToManyFields

[tognee]

This PR lets users set a ManyToManyField as a SynchronizedField without recieving the error:

Direct assignment to the forward side of a many-to-many set is prohibited. Use model_name.set() instead.

This does not solve the issue of whether a ManyToMany field should be translatable or syncronized, but gives the option for the user to syncronize it without errors.

This PR adds:

• the code changes to handle many to many fields
• a snippet model to test ManyToMany fields (with its migration)
• 3 unit tests to check if the SynchronizedField is respected correctly for ManyToMany

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.08%. Comparing base (420d774) to head (cb1d719).

Files with missing lines	Patch %	Lines
wagtail_localize/test/models.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
+ Coverage   93.07%   93.08%   +0.01%     
==========================================
  Files          47       47              
  Lines        4244     4254      +10     
  Branches      551      552       +1     
==========================================
+ Hits         3950     3960      +10     
  Misses        175      175              
  Partials      119      119              

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[tognee]

I've seen that bitnami moved all free postgresql images to bitnamilegacy. I changed it just to test the checks, but I think it could it be replaced with the official docker postgresql image