Update MusicBrainz URLs

The "mm" subdomain has long been retired and simply redirects to musicbrainz.org with no subdomain now, just as "www" does. Also, MB officially has the pages under https:// as its canonical URLs and with all the other good reasons of using HTTPS over HTTP, it just makes sense to use this for linking to MB.

See also chat in #musicbrainz-devel: http://chatlogs.musicbrainz.org/musicbrainz-devel/2014/2014-11/2014-11-02.html#T01-18-50-32205