* Rename sequences & indexes after renaming users table
* Migrate from auth.Group to a custom group model
* Delete original groups from auth_group table
* Update object & multi-object custom fields referencing the Group model
* Fix ContentType resolution
* Clean up obsolete logic for view/serializer resolution
* 12794 change User ref to get_user_model
* 12794 call get_user_model once in tests
* 12794 call get_user_model once in tests
* 12794 use settings.AUTH_USER_MODEL for FK reference