Cover Art Archive support for summary.html

Some talk on IRC for more background: http://chatlogs.musicbrainz.org/musicbrainz-devel/2014/2014-11/2014-11-26.html#T10-15-07-807376

Note that the complicated CAA querying will be useful for making a thickbox (or whatever) CAA gallery, per my comment on IRC.

Also, it would be good if nginx or something could be set up to cache CAA responses, so we won't have to send off a query whenever someone reloads a page or loads another recording MBID from the same release they were just at. :)