Disable power off button in GDM

[GreatEmerald]

As per #15 we now need to deal with GDM and its helpful power off button. This needs a yet another GConf database (the geoscripting one doesn't seem to be applied to the GDM user):

• Create /etc/dconf/db/gdm.d/99-site-settings:

  [org/gnome/login-screen]
  disable-restart-buttons=true

• Create /etc/dconf/profile/gdm:

user-db:user
system-db:gdm
file-db:/usr/share/gdm/greeter-dconf-defaults

• Run sudo dconf update && sudo systemctl restart gdm.service