Workflow modal must be closed in every Discourse tab

If the target user has more than one forum tab open, the modal appears in **every** tab,

and closing it in one tab does not dismiss the others. Worse, acting on one of the

redundant copies produces a misleading error instead of a no-op:

> An error occurred: The requested URL or resource could not be found.

So the user is told something failed when in fact they had already completed the step.