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).