Fix bug with Jellyfin mbrainz IDs being poor

Turns out Jellyfin uses a really esoteric form of Musicbrainz ID for
tracks. Instead of using the recording ID, it uses the specific hash for
a given version of a recording. A noble effrot to be specific, but we'd
much rather use Mopidy's recording ID when it's available.

Thus, we'll use Jellyfin's ID if that's all we have, but if we scrobble
the same track from Mopidy, overwrite the value.

vrobbler/apps/scrobbles/views.py

@@ -143,9 +143,10 @@ def jellyfin_websocket(request):
             video, request.user.id, jellyfin_data
         )
     if track:
-        # Prefer Jellyfin track IDs to Mopidy, so just overwrite anything in there
-        track.musicbrainz_id = data_dict.get(KEYS["TRACK_MB_ID"], None)
-        track.save()
+        # Prefer Mopidy MD IDs to Jellyfin, so skip if we already have one
+        if not track.musicbrainz_id:
+            track.musicbrainz_id = data_dict.get(KEYS["TRACK_MB_ID"], None)
+            track.save()
         scrobble = Scrobble.create_or_update_for_track(
             track, request.user.id, jellyfin_data
         )
@@ -195,10 +196,9 @@ def mopidy_websocket(request):
 
     scrobble = None
     if track:
-        # Mopidy MB ids suck, we'd prefer jellyfin, but we'll take this if we have no other
-        if not track.musicbrainz_id:
-            track.musicbrainz_id = data_dict.get("musicbrainz_track_id")
-            track.save()
+        # Jellyfin MB ids suck, so always overwrite with Mopidy if they're offering
+        track.musicbrainz_id = data_dict.get("musicbrainz_track_id")
+        track.save()
         scrobble = Scrobble.create_or_update_for_track(
             track, request.user.id, mopidy_data
         )