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.
Good point. I forgot that adding it to `systemPackages` only adds that package to PATH. This shouldn't be necessary, however, as the service's startup script has absolute paths to those package's files.

But wait, the ZNC package needs to be installed, right? That was my intention with adding it to `systemPackages` - to ensure it is installed for when the service tries to start it.