Subsonic API: expose more metadata in getAlbumList endpoint

Expose more data in the albumList endpoint to improve compatibility with Subsonic client apps.

Subsonic exposes this for album in the albumList endpoint:

    "albumList2": {
      "album": [
        {
          "id": "4489",
          "name": "Going for the One",
          "artist": "Yes",
          "artistId": "861",
          "coverArt": "al-4489",
          "songCount": 8,
          "duration": 2934,
          "playCount": 0,
          "created": "2018-11-24T14:27:06.000Z",
          "year": 1977,
          "genre": "Progressive rock"*
        }

Funkwhale 0.17 exposes this:

    "albumList2": {
      "album": [
        {
          "id": 2,
          "artistId": 2,
          "name": "Feels Like Home EP",
          "artist": "Becays",
          "created": "2018-11-25T15:36:40.634786Z",
          "coverArt": "al-2",
          "songCount": 25
        }

It would be nice to expose as much as possible of the "static" metadata: duration, year and genre. Some Subsonic clients (i.e. play:Sub) display and make use of this info.

playCount would be lower priority; I'm not aware of any Subsonic client app that makes use of this.