Support importing playlists from spotify (web frontend)

[Ethazeriel]

Having written the issue title I suppose this is a thing we already do (the entire point of the bot, in fact...) but I'd like to be able to see all of my spotify playlists in the web frontend, pick from the list to view all the tracks, then somehow add those to the queue.

I suppose this is actually several features wrapped into one;

• Acquire-from-web support
• Web endpoints using authenticated spotify requests
• new web track object that can work from just a TrackSource

I had forgotten this code entirely, but currently if we send the web parent a request to queue things the code is just

case 'queue':
  // yeah I have no idea what the fuck I want to do here
break;

We should probably figure that out.