[Remotescripts] Prevent *new* clients from running already executing remotescripts

[nothingface0]

Currently, when a remotescript starts executing, only clients which have already connected (i.e. users which have opened a remotescripts page) are updated with the fact that the script execution started.

When a new client connects (e.g. a new browser tab of the same script) and the script is already under execution, the client is able to press the Submit button.

There should be a common state stored somewhere for all WebSocket clients, so that any new client is updated with the current state upon page load.