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.

[quantenzitrone]

I recently had the problem, where Two artist share the same Deezer Page and after importing one release, I also added a wrong Spotify link to one of the artists. With the duplicate Deezer and wrong Spotify link, harmony kept giving me the wrong artist.
I only realized this after a few added releases, because MusicBrainz only shows the Artist Credit and not the artist name without opening the Artist Credit editing menu.

To prevent situations like this, I think harmony should warn if streaming platform artist links are attached to different (maybe even multiple) artists.

Specifically:
Probably every artist named "Bo" (specifically this Bo) as well as "BÖ" share the same deezer page.
Due to this release being credited to Bo on Deezer Harmony matched Bo instead of BÖ.
This could've been prevented by also checking the Spotify and iTunes (however iTunes links are country specific, so idk if that works) links for the artist, which would've been matched to BÖ.
After adding this release, I added BÖs spotify page to Bo.
After this, more mismatching could've been prevented by seeing that the Spotify link is attached to 2 different artists and warning.

[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.