Import of http://bugs.musicbrainz.org/ticket/5158

The current way the artist page is set makes it annoying to navigate the releases: you either see the RGs, but not what releases are in them, or the releases, but not in what RG they are. Allowing expanding the RGs to get the list of the releases they contain would be quite nice and useful. It might even allow to merge the Overview and the Releases tabs together, if we want to.