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. Testing Done: - Ran unit tests. - Used this in Review Board and tested the functionality of the WebHookTarget admin form.