Improve artist matching and combining

[kellnerd]

• Match individual artists (unordered) and not full artist credit arrays (ordered)
• Match release and track artists in merge algorithm
• Combine identifiers of all matched artists

This should solve multiple problems with consistent display of artists as linked entities and MBID resolving.

[kellnerd]

These shortcomings are also the reason why not all available artist links are seeded to the MB artist, as reported in #71.

[kellnerd]

Mostly done in bcd97ad, only matching of release and track artists remains as a potential improvement (to increase the chance of mapping sparse track artist data to an MBID already handled).

Having such an artist cache allows us to attach all available external identifiers to the artists and share them for the release and its tracks.
This is not limited to the merged, harmonized release: Enriching the source releases with external links allows us to display these for all alternative values as well.