Use playlists to privately share audio files (#2417)

Closes #2417 (closed)

need fixes :

• !2910 (merged)
• https://node2.funkwhale.test/library/playlists to be fixed (create playlists)
• add track to playlist is broken : sending this to the backend : https://node1.funkwhale.test/api/v2/playlists/undefined/add/
• return playlist.LibraryFollowbollean object in playlist endpoint
• by default we have a strich privacy level management : user.privacy_level will be followed before object.privacy_level. We probably should changes this and follow the less strict privacy level between object and user ? Or add a clear documentation and warnings around this ?
• /playlists/8c6662f6-2620-4bd5-8bf4-c62fc3419525 is broken, tracks are not displayed in the front
• resolve last code todos (see #2451 (closed))
• IntegrityError at /api/admin/music/upload/ update or delete on table "music_upload" violates foreign key constraint "music_upload_playlis_upload_id_9a68104f_fk_music_upl" on table "music_upload_playlist_libraries" DETAIL: Key (id)=(2) is still referenced from table "music_upload_playlist_libraries". (bulk delete on django admin pannel)
• user.privacy_level do not support followers, it is not clear which activities are concerned (#2362 (closed))
• playlist with privacy_level = follower fails to being created/updated