Allow user to listen to purchases from collection page

[chrislo]

It'd be great to allow users to directly listen to their music from /collection. What I'd really like to do here is introduce a persistant player that stays pinned to the bottom of the page while a user browses the site. That way we could replace the player in the Album pages with this new player and have the same one everywhere.

Some background

"distilled noise" talks about how a persistant player is really useful for DJs auditioning tracks by several artists and choosing what to buy here: https://www.youtube.com/watch?v=RQV-H0mF7E8