Add Wagtail 6 support

[zerolab]

Builds on #772

To-Do:

• modeladmin headers
• List locale in the core Snippet listing - separate PR

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7d9fc03) 92.73% compared to head (33b171b) 92.80%.
Report is 6 commits behind head on main.

Files	Patch %	Lines
wagtail_localize/views/edit_translation.py	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #776      +/-   ##
==========================================
+ Coverage   92.73%   92.80%   +0.06%     
==========================================
  Files          47       47              
  Lines        4130     4112      -18     
  Branches      603      599       -4     
==========================================
- Hits         3830     3816      -14     
- Misses        176      177       +1     
+ Partials      124      119       -5     

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