Deregister last app model does not delete the app

When using deregister to delete **User** and **Group**, the **auth** app still appear in the Dashboard but no more models are registered.

When all models off an apps are deregistered the app should be removed.

Start by tracing the deregister flow for User and Group and how the Dashboard determines which apps remain visible. Reproduce the case where the last models are removed, then verify that the auth app is no longer shown when no models remain registered.

Written by the indexing model from the issue text.