Fix passthrough for proxied stream metadata

* Make sure ProxyRoom metadata gets passed through to the listener if the
  stream's acceptSourceMeta flag is on.
* Expose acceptSourceMeta flag to admin
* FIXME: We're still not handling metadata 100% correctly...  We're using
  the current meta title as the stream title for new connections, when really
  that's probably just song information and we should always give our configured
  title as the "icy-name" header.