MBS-2225: Add support for collection subscriptions

http://tickets.musicbrainz.org/browse/MBS-2225

Tested manually in sandbox with http://leftmost.mbsandbox.org/collection/44d65377-7e31-4bcf-b248-59c02b1cef6a

(Note: When testing, a user's edits will not be displayed for them in /edits/subscribed or in subscription emails sent to them, as is the case with other entities.)