Use Meet API for collaborative editing

[bollwyvl]

The now-pre-released jupyterlab 3.1.0a7 (and now 8) includes the initial yjs-based implementation of collaborative editing.

The initial server is here:

• https://github.com/jupyterlab/jupyterlab/blob/master/jupyterlab/handlers/yjs_echo_ws.py

And the initial client connector:

• https://github.com/jupyterlab/jupyterlab/blob/16cba948f03a07578cb249c368ead7589b9cae6f/packages/docprovider/src/yprovider.ts#L19

We should be able to use the Meet API (a la #24) to re-implement this model, and allow documents shared with other members of the meeting.

A monkeypatchtastic approach:

• overload the request for a "real" websocket with mock-socket
  • fake the server behavior

A better approach would likely require an upstream change, and allow us to implement a JitsiProvider.