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 #715)

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

To-Do:

• [x] 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

```diff @@ 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](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail) | Coverage Δ | | |---|---|---| | [wagtail\_localize/modeladmin/options.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS9tb2RlbGFkbWluL29wdGlvbnMucHk=) | `93.93% <100.00%> (+0.60%)` | :arrow_up: | | [wagtail\_localize/modeladmin/views.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS9tb2RlbGFkbWluL3ZpZXdzLnB5) | `98.14% <100.00%> (+0.05%)` | :arrow_up: | | [wagtail\_localize/modeladmin/helpers.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS9tb2RlbGFkbWluL2hlbHBlcnMucHk=) | `97.43% <83.33%> (-2.57%)` | :arrow_down: | | [wagtail\_localize/test/settings.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS90ZXN0L3NldHRpbmdzLnB5) | `97.36% <80.00%> (-2.64%)` | :arrow_down: | | [wagtail\_localize/wagtail\_hooks.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS93YWd0YWlsX2hvb2tzLnB5) | `86.33% <58.82%> (-3.53%)` | :arrow_down: | | [wagtail\_localize/views/edit\_translation.py](https://app.codecov.io/gh/wagtail/wagtail-localize/pull/735?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wagtail#diff-d2FndGFpbF9sb2NhbGl6ZS92aWV3cy9lZGl0X3RyYW5zbGF0aW9uLnB5) | `88.68% <73.33%> (-0.97%)` | :arrow_down: |

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