Dedicated flex attrs for Deezer, Beatport album ID

- Similar to what the Spotify plugin does, on imports we save to a field
  `..._album_id` (spotify_album_id, deezer_album_id, beatport_album_id)
- It would be good to submit such a change to the 3rd-party plugins beetcamp
  and beatport4 as well (beatport_album_id, bandcamp_album_id).
- We might need to investigate why none of these
  flex attr fields get populated to the beets album level (`beet info -a`,
  album_attributes db table), it is only available at the item level (`beet
  info`, item_attributes db table). This should be tackled in a future
  issue/PR.

beetsplug/beatport.py

@@ -432,6 +432,7 @@ class BeatportPlugin(BeetsPlugin):
         tracks = [self._get_track_info(x) for x in release.tracks]
 
         return AlbumInfo(album=release.name, album_id=release.beatport_id,
+                         beatport_album_id=release.beatport_id,
                          artist=artist, artist_id=artist_id, tracks=tracks,
                          albumtype=release.category, va=va,
                          year=release.release_date.year,

beetsplug/deezer.py

@@ -98,6 +98,7 @@ class DeezerPlugin(MetadataSourcePlugin, BeetsPlugin):
         return AlbumInfo(
             album=album_data['title'],
             album_id=deezer_id,
+            deezer_album_id=deezer_id,
             artist=artist,
             artist_credit=self.get_artist([album_data['artist']])[0],
             artist_id=artist_id,