Introduce WebSockets #137
Description
The current session handling is anything but efficient. It is one of them main sources of our memory leaks (see #58).
Replace the current session handling with WebSockets and an appropriate subprotocol, e.g.: WAMP or JSON-RPC. WebSockets allows us to reliably detect client exits.
The first protoype should be quite simple. It should suffice if a WebSocket is opened and closed if a session (or project) is opened/closed. The backend should then be able to reliably close a mope project when a WebSocket is closed.