Enable "favoriting" of media

[Aaron Williamson]

Users should be able to mark media entries as favorites by clicking a button/link in the entry detail view (and probably from gallery views as well). This change will likely require the creation of a new "UserFavorite" model with a structure like:

{'user': user_id, 'media_entry': media_entry_id, 'created': datetime.datetime.now()}

...and also a new field on MediaEntry containing a count of UserFavorites for that entry (to avoid expensive queries to generate this number dynamically for every request).