Behaviour of power button in the Greeter is not consistent with the one in the regular GNOME session

Pressing the hardware power button when on the Tails greeter screen causes the computer to suspend instead of powering off and wiping the memory clean. Pressing the power button again shows the debian 9 lock screen.
I boot tails from an iso image on a usb flash written with dd.

Feature Branch: bugfix/13000-power-button-in-greeter

Related issues

Blocks #13234 (closed)

Original created by @cypherpunks on 13000 (Redmine)

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