MBS-4294: Add a "description" field to collections

http://tickets.musicbrainz.org/browse/MBS-4294

This adds a Description field to the collection edit page, and displays it (with an h2) in the collection page if it exists. I also added an h2 for "Releases" now that it won't be the only thing in the page.

I was thinking of adding a preview option, but that turned out to be less simple and it wasn't asked for in the original ticket anyway. If someone wants to add that though, feel free :)

See also: http://tickets.musicbrainz.org/browse/MBS-5809