Sleep and wake-up of inactive 'view's of a notebook.

This is a feature request.

After a browser session for the notebook is inactive for a long time, we shall put that window to sleep:
- the entire page is grayed out
- a pop up telling the user the session is sleeping and a button to wake it up.
- If the user clicks on 'wake up', the notebook is reloaded to the most recent content.

The main purpose is to minimize the risk overwriting changes the user done on the same notebook from another browser session. This happens quite often if the user works on more than one computers, or if the notebook is hosted on a cloud like environment via jupyterhub.

A similar feature was implemented in overleaf.com. I will try to upload a screenshot when I see one next time.