Playlist manipulation

[tsani]

We need an endpoint to allow clients to query the state of the playlist. Currently, the only supported operation is enqueuing tracks.

An interesting flow could be: 1. fetch playlist; 2. use interactive editor clientside to produce a subset of the tracks on it; 3. send transcode requests for each identified track; 4. send archival request for all transcoded tracks by this process; 5. download archive. This would be a nice workflow for music export (see #6).

• [x] get playlist
• [x] move track(s) in playlist
• [x] delete track(s) in playlist

[tsani]

Moving tracks is not explicitly implemented, but by using delete followed by add it can be simulated, barring any concurrency issues.