Fix Incorrect URL Links In Wagtail Based On Browser Language

[dchukhin]

One-line summary

When a user's profile in Wagtail (or their browser language) is set to an alias locale, some links in Wagtail incorrectly point to the alias locale.

Significant changes and points to review

• a new WagtailAdminRequestMiddleware tracks if the request is for a Wagtail admin URL
• AbstractSpringfieldCMSPage.get_url_parts() explicitly overrides the locale if 1. the request is for a Wagtail admin URL and 2. the locale does not match the page's locale (as a note, there is no public-facing behavior change here)
• a WAGTAIL_ADMIN_URL_PREFIX setting for the Wagtail admin prefix (no behavior change here, just setting this in settings, so it can be used in another file)
• tests for this behavior, including for the rich text block

Issue / Bugzilla link

Testing

Steps to recreate the bug:

1. Log in to Wagtail, go to /cms-admin/account/ → Language preferences → set preferred language to Portuguese (the pt-pt option) → Save.
2. Open the edit view of a published pt-BR page (for example, the pt-BR thanks page: http://localhost:8000/cms-admin/pages/466/edit/).
3. observe that the page is published
4. observe that the "PUBLICADA" link points to the pt-PT page (http://localhost:8000/pt-PT/thanks/) instead of the pt-BR page
5. Switch the account language back to English, and reload the pt-BR thanks page, and observe that the link correctly points to the pt-BR page (http://localhost:8000/pt-BR/thanks/)

To test the fix, follow the same steps, but observe that the "PUBLICADA" link points to the pt-BR page.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.34%. Comparing base (0958159) to head (ae63331).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
+ Coverage   79.26%   79.34%   +0.07%     
==========================================
  Files         154      154              
  Lines       10569    10592      +23     
==========================================
+ Hits         8378     8404      +26     
+ Misses       2191     2188       -3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.