data: Fix indirect conflict with exit.target via app.slice
systemd v247 now puts services into app.slice by default. But app.slice will have a conflict with exit.target, which in turn means that we still get a conflict with exit.target indirectly.
Fix it by adding the appropriate Slice=-.slice into gnome-session-restart-dbus.service. Also update the note in gnome-session-shutdown.service to point out the requirements.
Closes: #74 (closed)