Add ZNC module

I'm running a ZNC server on my NixOS machine. I'd like to move it into a module, which uses systemd to start and manage it.

How does this module look? I'm curious to hear about my creating a user and group, and about the uid and gid for those. Are we really managing a central list of these ids? That list says that there's a max of ~300 ids. Is that right? This doesn't seem like a sustainable thing.
systemPackages the paths in the package are merged in the system environment, which often not necessary. Does it start and work fine without systemPackages? Better :)