[urchatfm] Ensure hanging up and rejecting calls works consistently

[eamsden]

Call rejection and hanging up is an undertested function of urchatfm and switchboard. We should test that

Should work

• Hanging up on an ongoing call by either peer causes the other peer to exit the call.
• Hanging up by window closure can be detected (this may require some sort of heuristics in on-leave).
• A callee rejecting the call causes the caller to exit the call
• Exiting a call causes local media streams (camera/microphone) to be closed.
  [urchatfm] Close media streams when exiting a call #8

May need implementation

• A caller can hang up a ringing call
  Allow callers to cancel ringing calls #9