add user messages/notifications/progress updates

[7yl4r]

The server should send updates to the user via these built-in widgets in the top-right. This is best done using the websocket connection to send messages like "addNotification" from server to client. Will need to:

• Create a "sendMessage" and/or "sendNotification" method in py/game_logic/user/User.User
• write the javascript to add the message to the DOM
• add a message receiver to the js websocket parser for the new commands