Tracking issue: add systemd slices for related services

[h7x4]

Issue description

I'd like to start organizing some of the nixos services into systemd slices.

According to the systemd.slice docs, A slice unit is a concept for hierarchically managing resources of a group of processes. There are quite a few modules that split up their services into several systemd units, which I think makes sense to collect into a slice.

Some of the benefits of creating such slices would be:

• Easier for the user to configure resource limits for a system of services, e.g. nix hydra or prometheus with exporters.
• Cleaner and more organized output of systemctl status, indenting slice contents in subtrees
• Possibility to view interlaced logs from all units, using journalctl -u <slicename>.slice
• Easily declare dependencies for all units (e.g. when all workers of a service depend on something like postgres)

As a side note, I also think most slices should be prefixed with system- if there's not a good reason not to. By default, service and scope units are placed in system.slice. This would be in contrast to something like how php-fpm is currently put in a top-level slice (planning to move this as part of this tracking issue).

TODO

• [ ] services/backup/bacula.nix
• [x] services/continuous-integration/hydra
• [ ] services/desktops/cpupower-gui.nix
• [ ] hardware/video/nvidia.nix
• [ ] services/mail/sympa.nix
• [ ] services/misc/gitlab.nix
• [ ] services/misc/paperless.nix (#341278)
• [ ] services/misc/sourcehut/default.nix
• [ ] services/matrix/synapse.nix
• [ ] services/monitoring/cockpit.nix
• [ ] services/monitoring/graphite.nix
• [ ] services/monitoring/prometheus and services/monitoring/prometheus/exporters
• [ ] services/monitoring/ups.nix
• [ ] services/networking/jibri/default.nix
• [ ] services/networking/nncp.nix
• [ ] services/networking/softether.nix
• [ ] services/networking/wireguard.nix
• [ ] services/network-filesystems/moosefs.nix
• [x] services/network-filesystems/samba.nix (#279917)
• [x] services/security/clamav.nix
• [ ] services/security/esdm.nix
• [ ] services/security/kanidm.nix
• [ ] services/security/shibboleth-sp.nix
• [ ] services/system/cloud-init.nix
• [ ] services/system/kerberos/heimdal.nix
• [ ] services/web-servers/phpfpm, prefix slice with system-
• [ ] tasks/filesystems/zfs.nix
• [ ] virtualisation/digital-ocean-config.nix
• [ ] virtualisation/libvirtd.nix
• [ ] virtualisation/parallels-guest.nix
• [ ] virtualisation/vmware-host.nix

[tomodachi94]

Upstream PR filed at https://github.com/OpenPrinting/cups/pull/1035 to add a slice for the CUPS printing system, since it appears we use the upstream units.