Use ExternalReference to store MusicBrainz identifiers
Depends on tracker!107 (merged)
This merge request uses
ExternalReference to store MusicBrainz Identifiers instead of properties. According to @carlosg: "It is more rdf-y to use
ExternalReference than a bunch of properties to a same object"
I'm not sure how we should set the
source. Sources are named with the url from musicbrainz documentation: https://musicbrainz.org/doc/Recording, https://musicbrainz.org/doc/MusicBrainz_Identifier etc..