nixos/pleroma: add pleroma NixOS module

This module is not trying to configure either postgresql nor nginx.
It's is a design decision, not an omission. A webserver setup can be
highly complex.

The idea is trying not to be smarter than the user, providing them
with a simple tool. They are smart enough to figure the interaction
between the various component by themselves!

The module has one and only job: setting up a pleroma service.

modules/pleroma.nix

@@ -1,4 +1,4 @@
-{ config, options, lib, pkgs, stdenv, ... }:
+{ config, lib, pkgs, stdenv }:
 let
   cfg = config.services.pleroma;
 in {