View dedicated to artist

[orontee]

TODO

• [x] Add model for artist
• [x] Extend InformationService to identify image URL from Wikidata for a given Musicbrainz identifier for an artist
• [ ] Download artist image when entering a directory for an artist
• [ ] View with artist image, abstract, albums & tracks

Remarks

• Opportunity for a code cleanup: Move artist information from album model to the artist model.
• AFAICT Musicbrainz identifier are present only for artists from Mopidy-Local library
• Displaying discography would be nice
• Won't display artist image in parent directory since given a Ref of type ARTIST there's no cheap way to get the artist DTO! (See https://discourse.mopidy.com/t/retrieving-artist-details-from-an-artist-ref-through-http-api/5760)