The administration UI currently uses Django's default multi-object field selector widget for choosing groups and repositories. This implementation loads all objects at once on load and does not scale well on large implementations.
These changes replace the old selectors with a new, smarter objects selector widget. This widget has already been implemented in Review Board 2.5.7+ for selecting users. So, these changes involve removing the widget class from Review Board (and added to Djblets with Review Request 8845), adjusting the existing user selectors to use the moved class, and replacing old selectors to use the new selector. This includes groups and repository selectors in the administration UI.