improve playlist, cookies --> localStorage

Reopening player controls no longer restarts the video. This was a problem
with Chrome mobile. Its unpredictable page reloading also wiped the
playlist stored in window, so I switched to localStorage. It isn't
optimal, though. Now you can't have multiple playlists open
simultaneously.
TODO: explore using state objects for playlist storage instead, assuming
they don't get wiped when Chrome decides to reload the page.

As the server didn't use volume or sorting cookies, localStorage was a
better alternative. Volume cookies would make sense, though.