Terminate GDM's GNOME session after the amnesia user logs in, in order to free memory (refs: #12092)

I've heard rumors that we can drop this hack when we switch to Wayland (#12213).
We'll see :)

We kill it as part of desktop.target, i.e. during the "Applications" phase of
the initialization of the GNOME session. We cannot do this earlier reliably:

 - basic.target is started by "systemd --user" for almost every command run as
   the amnesia user and may thus be triggered too early, at a time when we still
   need GDM's processes.

 - If we do this as part of basic.target, it sometimes happens before amnesia's
   X.Org has started, and sometimes after that, which causes racy behaviour,
   weird bugs, and amnesia's $DISPLAY can be either :0 or :1, which breaks our
   code that relies on that value to be always the same.

We're in no rush to kill GDM's GNOME session super early anyway.

Note that we keep GDM running while we kill its GNOME session,
otherwise, the amnesia user can't unlock the screen:

  Failed to open reauthentication channel: Gio:DBusError:
  GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name
  org.gnome.DisplayManager was not provided by any .service files

Also, we ensure gdm-session-worker does not start new sessions once the amnesia
user has logged in, which should hopefully prevent GDM from activating
such a session while we want the amnesia's user session to remain active.