MBS-7399: Allow linking to artists on TheSession.org.

Replaces the matching anything (`[^\/]+`) with matching against a
whitelist, to allow for the nesting of `recordings/artists`.
Tidy up the thesession.org URLCleanup.

Includes filtering out unwanted cruft at the end of the URL.
Merge remote-tracking branch 'freso/mbs-7399-thesession-artist-urls' into beta

* freso/mbs-7399-thesession-artist-urls:
  Tidy up the thesession.org URLCleanup.
  MBS-7399: Allow linking to artists on TheSession.org.