CAA types i18n (re-do)

Related to a bunch of cover-art/i18n-related bugs, and I'm sure I've missed some, but: http://tickets.musicbrainz.org/browse/MBS-5776 http://tickets.musicbrainz.org/browse/MBS-5432, probably http://tickets.musicbrainz.org/browse/MBS-5567 as well.

Reworks a lot of our cover art stuff to support i18n. Specifically: gets rid of a lot of the "FIXME: Replace with a real Entity::Artwork object" by replacing them with Entity::Artwork objects, as well as using Artwork->find_by_release (and a subsequent CoverArtType load) rather than CoverArtArchive->find_available_artwork (i.e., using Entity::Artwork).

I haven't done this here, but we may want to consider moving cover art types to the in-memory cache (along with artist types, languages, countries, etc.).