Ensure that the emergency shutdown works while the screen is locked

The following sentence might pretend that the locked screen interacts badly with the emergency shutdown:

"When the screen is locked, it’s not possible to quickly shutdown the computer properly (i.e. with memory erasure): the computer’s shutdown button is inhibited, and the fact to put out Tails’ device does not shut the computer down."

Parent Task: #5684 (closed)

Original created by @tails on 6017 (Redmine)