Miscellaneous updates: add series, add search_areas, update get_releases_by_discid

Just some low-hanging fruit.

Series and area stuff is of course already shipped; the get_releases_by_discid has to do with https://bitbucket.org/metabrainz/musicbrainz-server/pull-request/1100/mbs-7757-mbs-7758-allow-lookup-via-the which is only on beta until the 4th, but is happening soon in any case.

Not sure if there's tickets, if there are someone can point me to them etc. and I'll update CHANGES.
Is `all` the only allowed value or can this be used to browse releases of a specific other media type as well?