Implement Call Hangup

This implements call hangup into Talkilla, part of https://trello.com/c/Jg1wWn9x. Specifically:

- The server now handles call_hangup messages
- The worker can handle the call_hangup message from the server, as well as sending the messages to the server.
- The hangup button on the UI is now enabled
- The call model now stores id and otherUser, rather than caller and callee. As this is clarifies the values with respect to the differences of incoming and outgoing calls.
- Calling hangup on the call model will reset it to the 'ready' state. This is more expected for future functionality where ending a call may leave the window open.
- The webrtc model now closes the peer connection on hangup

The code also limits the user to only make one call (aka chat window) at a time, until we implement multiple calling. This is to ensure that the currentCall data is kept up to date so that calls can be hung up cleanly.