playout history

* Schema change (WIP): Dropped required foreign key constraint on
  cc_webstream_metadata because it doesn't have to be associated with
  a webstream now.
* Fixed annoying extra comma in Now Playing/dashboard view at the top.
* Relay Icecast metadata updates received by Liquidsoap from live
  connections (master/show source) over to Airtime, using the same code
  that relays webstream metadata. Store that metadata in the
  cc_webstream table and adapt it to work.
* Updated APIs to show this logged live metadata during live shows.
  (Works with Radio Page, embedded player, dashboard, etc. now!)