Revert "Piece and Playlist: use a different channel for model and collection."

This reverts commit 38a10eba52ec49a17bf338b606bf8b5570ee6336.
Media.Playlist: adds reverse relation from Piece to Playlist

This is needed to avoid erroneous updates as we instantiate various
Piece.Collection, one for every Playlist.

Each one of them is initialized with the set of pieces that belong to
the playlist. Up to that point everything so far works.

However when we add a new piece the backend broadcasts a 'create' event
and every Piece.Collection reacts to it, and so the piece ends up
everywhere.

Adding this reverseRelation (and  includeInJSON: '_id') avoids that.