Add Video button to the conversation views

This change is part of https://trello.com/c/BVa0jV7f to allow the caller to view conversation history before starting video and text calls.

It changes the conversation window so that it doesn't create a call straight away, but provides video and audio buttons, to start the call when the user is ready.

The audio button does work, but at the receiving end the assumption is that it is still a video and audio call. We'll fix that on a later card.

Specific changes:

- `ChatView` has been renamed `ConversationView`
- The peer data now gets stored in a User object in the ChatApp. The use of the ChatApp is temporary whilst we possibly consider a conversation model object which would hold both the peer, the user's data and maybe other items.
- Creates a `CallControlsView`. The new view exclusively handles the call control buttons, showing them at appropriate times.
- `onConversationOpen` no longer starts the call, the `CallControlsView` now does this.