Port to Django 5.1

Django 5.1 is currently in experimental, so it would be a good idea to port to it so that we aren't taken by surprise by CI failures when it lands in testing.

So far it doesn't look too bad, but I've found the following things that need to be fixed:

Set default value for ArrayAgg for Workspace.user_roles (deprecated in 4.0; !1491 (merged))
Log out via POST rather than GET (deprecated in 4.1; !1488 (merged))
Update stray uses of assertQuerysetEqual (deprecated in 4.2; !1491 (merged))

Edited Mar 26, 2025 by Colin Watson

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