Add Wagtail 5.2 compatibility

[zerolab]

This PR fixes #734, supersedes #733 and adds compatibility with Wagtail 5.2. It:

• drops the unused LocalizedPreviewSidePanel
• moves side panel definitions in edit_translation
• adds the side panel for snippets (Wagtail 5.1+ only)
• Fixes deprecation warnings around permissions
• Moves the "Source > Translation(s)" breadcrumb bar out of React and into the template (a minor step towards Drop React #715)

Preliminary build:
wagtail_localize-1.7b1.tar.gz
wagtail_localize-1.7b2.tar.gz

To-Do:

• use conditional templates for pre 5.1, and 5.1+ because of {% dropdown %}

[codecov-commenter]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (5b435d9) 92.73% compared to head (354088f) 92.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
- Coverage   92.73%   92.60%   -0.14%     
==========================================
  Files          47       46       -1     
  Lines        3966     4002      +36     
  Branches      588      596       +8     
==========================================
+ Hits         3678     3706      +28     
- Misses        174      176       +2     
- Partials      114      120       +6     

Files	Coverage Δ
wagtail_localize/modeladmin/options.py	93.93% <100.00%> (+0.60%)	⬆️
wagtail_localize/modeladmin/views.py	98.14% <100.00%> (+0.05%)	⬆️
wagtail_localize/modeladmin/helpers.py	97.43% <83.33%> (-2.57%)	⬇️
wagtail_localize/test/settings.py	97.36% <80.00%> (-2.64%)	⬇️
wagtail_localize/wagtail_hooks.py	86.33% <58.82%> (-3.53%)	⬇️
wagtail_localize/views/edit_translation.py	88.68% <73.33%> (-0.97%)	⬇️

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