Use the actual IDs for Musicbrainz resources

We were generating internal unique IDs for Musicbrainz resources,
which looked like this:

    urn:ExternalReference:https:%2F%2Fmusicbrainz.org%2Fdoc%2FArtist:76b2e842-5e85-4c97-ab62-d5bc315595b5

The actual Musicbrainz URL is already guaranteed to uniquely represent
the resource, so let's use that to identify the resource inside Tracker.
This means the URL is shorter, more readable, and can be browsed. The
above example becomes this:

    https://musicbrainz.org/artist/76b2e842-5e85-4c97-ab62-d5bc315595b5

This shouldn't be backported to the stable branch.
1 job for sam/musicbrainz-urls in 1 minute and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #524848
test-fedora-latest

00:01:42

 
Name Stage Failure
failed
test-fedora-latest Test
FAILED (errors=1)
-------

Full log written to /builds/GNOME/tracker-miners/build/meson-logs/testlog.txt
A connection to the bus can't be made
Uploading artifacts...
build/meson-logs/*.txt: found 2 matching files
Uploading artifacts to coordinator... ok
id=524848 responseStatus=201 Created token=PUUsG_xU
ERROR: Job failed: exit code 1