Get visual feedback about notebook running status

[disberd]

As discussed on zulip, it would be very helpful to see which cells are running on the notebook on the server especially for long running computations.

Another interesting point raised on discourse would be having the possibility of relaying logs back to the clients, though I am not sure how feasible or easy that is.

Tagging @ctrekker as he mentioned the first part would be rather simple to implement.

[ctrekker]

Linking #64 and fonsp/Pluto.jl#2130