Update demo site for development

[nickmoreton]

When merged this will:

• Update the codebase to support Wagtail 4.1 to 5.2 and Django 4.1 to 4.2 to keep it inline with the work here: https://github.com/longclawshop/longclaw/pull/489
• It's not ready for a production deployment yet
• It's using the branch here: https://github.com/nickmoreton/longclaw/tree/modernise/testing as a package source for the moment
• Please ignore the rubbish content for the moment 😄
• I think it will help with development as well to work out what isn't working since updating the package repo.
• It can be run in docker and use the local copy of the modified package.