Previous "Restart" button shoult "Shutdown" in new Greeter

The “Restart” or “Shutdown” button in the top-left corner of the new Greeter proposes to restart the system. It has been added because, once in this state, users don’t have a proper way of restarting of shutting down their computer (apart from forcing a shutdown mechanically). But then, the option that makes sense is “Shutdown” (if they want to restart on their usual operating system) and not restart (why would they want to restart Tails if they were still in the Greeter?).

Parent Task: #10828 (closed)

Original created by @sajolida on 11955 (Redmine)