Resolve multiple MBIDs with one API request

[kellnerd]

Now that MBS-13943 has been implemented, multiple URLs can be looked up at once.
Harmony's MBID mapper should make use of it, this is going to significantly speed MBID resolving.
This is going to require a significant refactoring.

P.S. It is probably feasible to start checking for existing release URLs as well, it should be scalable now when more providers get added.

[kellnerd]

I forgot to close this, but this was done in 88baf79.

Release links are also looked up as part of the same request now (ca5ac5b) and a warning is shown for each potential duplicate on MB (683d447).