Upgrade Wagtail to 7.3

[JakubMastalerz]

Ticket: https://torchbox.atlassian.net/browse/TSE-117

Wagtail 7.3 release notes

Add a link to the commit or file for each consideration if you made any code changes.

Code reviewing a consideration.

Use the tick box to indicate a consideration has been code reviewed as OK

Upgrade considerations - changes affecting all projects

• Loading of fixtures containing models referencing Locale
• New image quality defaults

Upgrade considerations - changes affecting Wagtail customizations

• Userbar items rendering

Upgrade considerations - changes to undocumented internals

• Userbar items internals
• Image URL generator changes
• Refactored validation logic for generic model CreateView and EditView

All versions extras

Identified by the support team as additional considerations to be checked for all Wagtail upgrades

• Node version should be >=20
• Dockerfile images should be at least python 3.13 and the OS should be pinned.
• TBX forms should be updated to the latest verion if it's used.
• Are any changes required around Wagtail Footnotes? Have you tested the performance when saving pages with a high number of footnotes?
• Django has been updated to the latest 4.2 or 5.2 patch version.
• If an upgrading.md file exists, update it with the latest information for any future upgrade checks.
• Overridden admin templates have been updated, if required, to match the latest Wagtail version.
• Are any wagtail packages using our forked version tags?
• Run trivy to check for security patches that poetry may not bump.
• CHANGELOG updated or created.

[damwaingames]

Looks good to me