Wagtail 4.0 compat

[zerolab]

This PR replaces #602 and finalizes 4.0 compat, with a few of changes:

1. Wagtail 4.0 compat #602 added the preview side panel, however that fails as the preview panel JS works with the regular editor form
2. the non-page interface was broken as it relied on page perms etc.
3. updated the localize initial migration to vary the Wagtail core migration depending on what Wagtail version is installed. This is only relevant for new installs and the test suite.
4. ensuring Wagtail 4.0 is added to the tox test targets

[codecov-commenter]

Codecov Report

Merging #613 (cf4f115) into main (a9f047e) will decrease coverage by 0.28%.
The diff coverage is 56.25%.

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
- Coverage   91.40%   91.12%   -0.29%     
==========================================
  Files          47       48       +1     
  Lines        3921     3953      +32     
  Branches      598      602       +4     
==========================================
+ Hits         3584     3602      +18     
- Misses        192      205      +13     
- Partials      145      146       +1     

Impacted Files	Coverage Δ
wagtail_localize/side_panels.py	50.00% <50.00%> (ø)
wagtail_localize/views/edit_translation.py	84.71% <64.28%> (-0.55%)	⬇️
wagtail_localize/compat.py	51.85% <0.00%> (ø)
wagtail_localize/wagtail_hooks.py	78.26% <0.00%> (ø)

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