Feature request: Show if a notebook is open from multiple clients

[BioTurboNick]

With a multi-user Pluto server, it can be great to see the same code running live with someone remotely. But, sometimes multiple people can be unknowingly using the same notebook at the same time. Would be nice to have some notice somewhere while viewing a notebook that a notebook is being interacted with from multiple clients. Something unobtrusive and just an indicator, to give a heads-up that spooky things could start happening.

[Pangoraw]

FYI, there are plans to make the multi-users story more robust in the future (see #1729 and #2296).