Disable save actions in the frontend

[ellisonbg]

Right now, if a user hits the keyboard shortcut to save a document (Cmd+S) it shows a dialog indicating the file has changed on disk:

I think we should disable all save actions/shortcuts in the frontend as a manual save isn't really a thing with jupyter server documents (all handled by the YRooms and loaders in the server).

[ellisonbg]

This is fixed in main.