[front] add cookie support in radio manager

The manager now sets a cookie when playing a track, and retrieves it
when loading the playlist, to start where the user left off.

* using cookie-js.
* The manager is not responsible for knowing if a new Track playing is
  in the radio playlist, or somewhere else. It's the current controller
  that decides if it plays radio tracks or not (default behaviour is
  that it's a radio track).
* Incidentally, the radio manager doesn't fire Radio:newTrackPlaying
  events, it's the current controller that does (it feels weird though).