As noted by 5bf4e3d92f, MusicBrainz
external IDs (`*_album_id`) were only saved for items and not albums.
This commit addresses that by copying `AlbumInfo` fields to the `Album`,
i.e. what's saved in the DB.
This is similar to how `TrackInfo` fields are copied to `Item` instances
except the copying is done at a different time since we only get an
`Album` much later in the import flow.