Refactor scrobbling code and add Music

If you send Track data from the Jellyfin Webhook plugin, we'll do the
right thing with it. Lots more to do to clean this up, but it also
involved moduralizing the code for scrobbling so it's a little simpler
to understand what's going on.

vrobbler/apps/music/constants.py

@@ -0,0 +1,16 @@
+JELLYFIN_POST_KEYS = {
+    'ITEM_TYPE': 'ItemType',
+    'RUN_TIME_TICKS': 'RunTimeTicks',
+    'RUN_TIME': 'RunTime',
+    'TITLE': 'Name',
+    'TIMESTAMP': 'UtcTimestamp',
+    'YEAR': 'Year',
+    'PLAYBACK_POSITION_TICKS': 'PlaybackPositionTicks',
+    'PLAYBACK_POSITION': 'PlaybackPosition',
+    'ARTIST_MB_ID': 'Provider_musicbrainzartist',
+    'ALBUM_MB_ID': 'Provider_musicbrainzalbum',
+    'RELEASEGROUP_MB_ID': 'Provider_musicbrainzreleasegroup',
+    'TRACK_MB_ID': 'Provider_musicbrainztrack',
+    'ALBUM_NAME': 'Album',
+    'ARTIST_NAME': 'Artist',
+}