data: Overhaul systemd units

data: Overhaul systemd units

This simplifies the structure of the systemd units, since we no longer
support running with X11. But it also allows us to pass in the
gnome-shell session mode via systemd.

This removes the need for GDM to have any special code to handle the
GNOME_SHELL_SESSION_MODE environment variable, and allows GNOME sessions
to specify directly what session mode they need without the involvement
of GDM. This makes it trivial to define custom gnome-session sessions
that use custom gnome-shell session modes.

Sessions that consist entirely of a gnome-shell mode (like the
gnome-classic session) will need to do a little more legwork to define
themselves as proper gnome-session sessions.