Playlist player should support many copies of same track

[sudara]

Multiple versions of the same track added to a playlist can't be navigated to from one another in the sidebar playlist

Not sure if this was possible before #1087.

Basically, once you click a track in the sidebar, you can no longer click another track.

Currently playlist urls look like so:

http://localhost:3000/marieh/playlists/raclette/baguette-laonnaise

so there's no way to link to anything other than the first track of the same asset without changing that url scheme.

Playlist ajax requests use a custom show_track_in_playlist route that routes to playlists#show.