WIP: switch to using systemd --user

Iain Lane requested to merge wip/laney/systemd-user-switch into master

This switched GDM to being launched by systemd --user, via gnome-session-systemd.

I have not converted the gnome-initial-setup or other sessions yet. I'd rather get this out of the door and work on that later - but perhaps someone could check if those are at least not broken by these changes.

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