AppRTC: Use WebSockets instead of Channel API.

With this change no messages are stored on GAE. Instead, they are stored on Collider.
GAE still keeps track of which users are connected to a given room.
Note that loopback will no longer work.
Are you going to fix loopback later? It will be a blocking issue to push this to prod.
I think we can fix it by mimicing a fake client in the same page, say
if (loopback) {
  var fakeClientSocket = new WebSocket(...)
  fakeClientSocket.onopen = function() {
     // register as a different client, say 'loopbackfake', in the same room
  }
fakeClientSocket.onmessage = function() {
  // if it's offer, change type to answer, and pass to the peerconnection
}
}