PICARD-662: only consider collections of the "Release" type

After 2014-11-17 schema change, collection types were added by MBS-1059, causing an exception.
Hm, this isn't quite right. There's also several subtypes of the "Release" type, specifically "Owned music" and "Wishlist" -- they're underused right now because a bug means existing collections can't be changed to these subtypes. Possibly we should add something that lists the entity type explicitly, but a solution that'd work without that is checking which subelements are available -- if there's a release-list element, it's a release collection, if there's an event-list element it's an event collection.