Allow tracks to be associated with a YouTube video

[EventideGlow]

Requested by AnimatronicPony. The idea is that, when an artist creates a video that goes with their track, Pony.fm's listeners would be able to enjoy that video while listening to the track.

• an artist would be able to associate a YouTube video with their track
• for tracks that have an associated video, the video would replace the cover art on track pages; these pages may also have a somewhat different layout that better showcases the video
• when you play the track, the audio will come from Pony.fm but it'll also play the video, synchronized to the same time
• the video itself will not have its own playback controls and its audio will be muted
• the video will only be loaded on track pages, and will be unloaded if you leave them #performance