Updated playlist management API

This is a cleanup of the /playlists API endpoints, in particular:

• removal of the /playlist-tracks endpoints
• moved all the removed /playlist-tracks actions such as moving a track in the playlist or deleting it to playlist/{id}/move and playlist/{id}/remove
• Don't expose playlist tracks ids anymore. We identify them by their index in the playlist

This makes the API more consistent and easier to deal with and document.

cc @Sporiff