MBS-3971, MBS-3810: Group relationships underneath tracks

This replaces the "(Show|Hide) Credits" link on release pages with a link that'll switch between (1) displaying track relationships inline (default) and (2) displaying track/release relationships underneath the track listing.

example:
http://bitmap.mbsandbox.org/release/61bffce0-2af5-3486-9062-0174628423c3

I don't know if the show subroutine in Controller::Release is the best place to dump all that code, but I'm not sure where else to put it.

http://tickets.musicbrainz.org/browse/MBS-3971
http://tickets.musicbrainz.org/browse/MBS-3810

I mostly performed manual testing, but also ran the Release::Show and Release::Details tests.