Improve collections feature

by Sean Burke for MetaBrainz Foundation Inc.

The primary goal of this project is to implement the collections feature in such a way that a given user is able to create an arbitrary number of lists of releases. In doing so, the user may create lists to suit their own purposes: Albums I Own, To Buy, Sorted and Tagged, etc. A given release could be included on any number of these lists. Simple access control will also be implemented, allowing users to choose whether a list is publicly accessible or private.