scheduler: add a systemd slice

scheduler: add a systemd slice

From the systemd documentation[1]:

> A slice unit is a concept for hierarchically managing resources of a group of processes.

Benefits of collecting our two systemd services into a slice include
* ease of configuring resource limits on the entire CUPS system,
* ``systemctl status`` showing the CUPS units in an indented subtree,
  making it more organized,
* and the possibility of viewing all interlaced logs from all of the CUPS
  daemons using ``journalctl -u system-cups.slice``.

[1]:
https://www.freedesktop.org/software/systemd/man/latest/systemd.slice.html
or ``man systemd.slice(5)``