Translatable ModelAdmin

[dinoperovic]

This PR adds support for translating pages and models registered using Wagtail's modeladmin.
It mostly re-uses the snippets approach and exposes the TranslatableModelAdmin class.

Translatable ModelAdmin has been a requirement in my personal projects and I have been using a similar setup that works pretty well.

Fixes #291.

A less intrusive approach that I was considering is to release a separate wagtail-localize-modeladmin package. In
this case the snippet views can be "patched" by placing the app above wagtail.snippets and handling redirects to modeladmin instances when applicable.

Curious to hear your thoughts and plans for supporting ModelAdmin localization.

[zerolab]

Thank you for this @dinoperovic
Had a cursory look and it feels the views side of things (or part of it) could be addressed by wagtail/wagtail#7857. Can you double check?

Will aim to get some review time next week.

[dinoperovic]

@zerolab Yeah I've missed that one. It looks like this could replace some parts of view logic. The views would still need to be overridden to display translation pages etc. Also, to support the previous Wagtail versions. It would be a good idea to align with the approach in the wagtail/wagtail#7857 once it's accepted.

I can take a closer look next week.

[zerolab]

cc @kaedroho (in the context of modeladmin/snippets thinking)

[codecov-commenter]

Codecov Report

Merging #550 (abbac64) into main (8278ab9) will increase coverage by 0.25%.
The diff coverage is 95.89%.

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
+ Coverage   91.41%   91.66%   +0.25%     
==========================================
  Files          37       42       +5     
  Lines        3263     3457     +194     
  Branches      529      557      +28     
==========================================
+ Hits         2983     3169     +186     
- Misses        161      165       +4     
- Partials      119      123       +4     

Impacted Files	Coverage Δ
wagtail_localize/wagtail_hooks.py	80.11% <ø> (ø)
wagtail_localize/models.py	95.85% <33.33%> (-0.28%)	⬇️
wagtail_localize/views/edit_translation.py	86.55% <33.33%> (-0.68%)	⬇️
wagtail_localize/modeladmin/views.py	97.53% <97.53%> (ø)
wagtail_localize/modeladmin/__init__.py	100.00% <100.00%> (ø)
wagtail_localize/modeladmin/helpers.py	100.00% <100.00%> (ø)
wagtail_localize/modeladmin/options.py	100.00% <100.00%> (ø)
wagtail_localize/test/models.py	97.56% <100.00%> (+0.02%)	⬆️
wagtail_localize/test/wagtail_hooks.py	100.00% <100.00%> (ø)
wagtail_localize/views/submit_translations.py	94.76% <100.00%> (+0.68%)	⬆️
... and 1 more

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[zerolab]

Hey @dinoperovic,

This is a lot of excellent work, thank you.
Left a number of questions and comments.

Additionally, this could really use some documentation

[dinoperovic]

Hey @zerolab,

I've made the requested changes, still missing the documentation which I will look to add.

[kaedroho]

This is excellent work, thanks! Just one minor thing I wanted to question

[zerolab]

Merged. Thank you @dinoperovic. Will make a new release sometime towards the end of the week