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.
We need to check 'channelReady' (probably rename to socketReady) to decide if we should call websocket.send, or xhr post, which means we also need a template var for the GCE https server url.