We're running into some trouble that the old django-multiselectfield

project is unmaintained and does not support Django 4.x+. There's a

third-party fork which has had some updates, but no releases of that

that we can consume.
django-multiselectfield itself isn't very big, and of what it offers, we

don't actually use very much. This change adds a new field type to

Djblets to do the database serialization/deserialization to join and

split comma-separated values. It doesn't support the validation that

django-multiselectfield does, but we don't actually care about that

functionality.

Ran unit tests.
Used this in Review Board and tested the functionality of the

  WebHookTarget admin form.