mbs-7934: IMSLP for artists
mbs-7934: improved validation, fixed dumb mistakes
Merge remote-tracking branch 'reosarevok/mbs-7934' into beta

* reosarevok/mbs-7934:
  mbs-7934: improved validation, fixed dumb mistakes
  mbs-7934: IMSLP for artists
Restructure URL Cleanup/type guessing tests to actually get run; also, add an IMSLP guess-type test for artists.