Add affordance to close errant staffing request

[marcua]

Today, the Staff/Restaff buttons allow you to create a new staffing request for a task.

They look like this in the Team Info card:

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And like this on the project management screen

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If you click the buttons, they end up looking like this:

One issue project managers have is that they sometimes accidentally staff a task. There is no affordance to undo that staffing. To facilitate this, we can do the following

• If there is an open staffing request, persist the StaffBot request sent message across page refreshes (it doesn't currently persist). Change the text to Close StaffBot request.
• Make the Close StaffBot request button wrap the already existing close_open_staffbot_requests logic (we likely need to add a view for this). Clicking that button would close all open requests, and also update the interface to go back to an original-state Staff/Restaff button.