Add a Chat Window

[rameshvarun]

If players are able to play with strangers, we probably want some kind of chat system that is available even before the game actually starts. We already have data channels open so we might as well allow chat messages to be sent over these. Also, because it's WebRTC we could eventually support audio chat.

For now just give players random usernames. Give the host the ability to disable chat, and allow the chat window to be hidden / shown at any time.