Prevent users from closing Tails Upgrader while an upgrade is being applied
The current dialog when the upgrade is being applied has an "OK" button that, when clicked closes the dialog.
The upgrade is still being applied in the background: tails-upgrader is still running and the USB stick is flashing.
We should prevent people from closing this dialog until the upgrade is fully applied.
At this point, Tails is offline and we recommended people to close everything else. People click OK might wonder what should happen next and we won't display anything else before several minutes. I guess that a lot of people might restart in doubt of what's happening.
I guess that this is the root cause behind #14754.
Marking as Elevated as it might lead to a broken Tails.