Rephrase shutdown message

The current shutdown message is problematic in many ways:

• It’s confusing about the need to remove the USB stick (#7617 (closed)):
  • “I didn’t remove the USB stick in time! Will this leave unexpected data somewhere!”
  • “Do I really have to unplug the USB stick if I want to restart Tails?”
• The message about memory wipe is hard to understand:
  • “If the memory wipe fails then I’m leaving traces on the computer.”
• We could also suggest forcing the shutdown.

Parent Task: #5417 (closed)

Subtasks

• #7617 (closed)

Original created by @sajolida on 10645 (Redmine)