When a user closes an issue that is blocked by another, the "Are you
sure you want to close this blocked issue?" modal displays but does not
go away after clicking "Yes, close issue". This causes confusion in two
ways:

1. It's not clear whether the button was clicked.
2. When the issue is closed, the modal still remains, making it look
like the issue may not have been closed.

We now hide the modal immediately after the "Yes" button is clicked.
Note this only happened if the bottom `Close issue` were clicked; the
top one worked fine.