bug: postfix service doesn't notice changes to mailing list memberships

[jfly]

Relates to #485.

#495 adds simple-nixos-mailserver to umbriel, but it has a bug: when you change mailing list memberships, postfix doesn't notice the change.

postfix doesn't actually need to get reloaded to observe this change, but we do need to re-run postmap to generate updated db map files. This happens in the postfix-setup service.

This doesn't play nicely with how we're using a sops-nix template to generate the map file, because the path never changes, which means the postfix-setup service never re-runs.

I discussed this with @Mic92, and fixing this requires a change to sops-nix. We have two ideas:

1. Feature request: "input addressed secret paths" Mic92/sops-nix#648 would help here, but is currently blocked by nix getting support for parsing yaml
2. Feature request: support restartUnits and reloadUnits for templates Mic92/sops-nix#634

I'm going to work on 2) now.