Log Message:
Update wagtail to 2.12.4:

 * Fix: CVE-2021-29434 - fix improper validation of URLs ('Cross-site \ 
Scripting') in rich text fields (Kevin Breen, Matt Westcott)
 * Fix: Reverse migration errors in images and documents (Mike Brown)
 * Fix: Avoid wagtailembeds migration failure on MySQL 8.0.13+ (Matt Westcott)
 * Fix: Un-pin django-treebeard following upstream fix for migration issue (Matt \ 
Westcott)
 * Fix: Prevent crash when copying an alias page (Karl Hobley)
 * Fix: Prevent errors on page editing after changing LANGUAGE_CODE (Matt Westcott)
 * Fix: Correctly handle model inheritance and `ClusterableModel` on \ 
`copy_for_translation` (Karl Hobley)