Only allow one call to happen at any time for now
Remove unusued call handling code from app.port.
Add a call_hangup message to the server
Add call hangup function to the WebRTCCall model
Add worker handling for receiving a call_hangup message from the app
Make the worker listen to the call_hangup messages from the server and handle them appropriately
Initial hooks for handling sending of hangup when the call window closes, and receiving at the other end