Start Y Websocket Server earlier and make it resilient to crashes

[Zsailer]

This prevents the server locking we see in #290

This PR makes two critical changes:

1. Instead of starting the y-websocket server when the first YWebsocketHandler request is made, this PR starts the server in the startup sequence of the ExtensionApp.
2. Handle exceptions in the ywebsocket server using the exception logging offered in Add WebsocketServer exception handler y-crdt/pycrdt-websocket#31

This depends on this PR in Jupyter Server: jupyter-server/jupyter_server#1417

[github-actions]

👈 Launch a Binder on branch Zsailer/jupyter-collaboration/resilient-y-server

[davidbrochart]

Thanks @Zsailer.
I think that it would be better to have an auto_restart parameter passed to pycrdt-websocket's WebsocketServer, that would essentially do the same as here (restart on exception). That way, it would benefit to all users of the WebSocket server, not just jupyter-collaboration. What do you think?
I can open a PR there if you want.

[Zsailer]

I think that it would be better to have an auto_restart parameter passed to pycrdt-websocket's WebsocketServer

Sure! That sounds great. I'm happy to make the PR over there as well. Just let me know.

[davidbrochart]

I opened y-crdt/pycrdt-websocket#31.

[Zsailer]

This depends on a release of pycrdt-websocket (hence why the tests are failing).

[Zsailer]

Closing in favor of #295