Restructure folders, add mail host

The entire file structure has been reorganised into stable and unstable
to separate deployments running on either channel. A `mail` host running
Simple Nix Mailserver has also been added for testing to see if it's a
good alternative to soverin as well as SMTP for other services.

config/hosts/rudiger/services/nginx.nix

@@ -1,8 +0,0 @@
-{ ... }:
-{
-  imports = [ ../../../common/services/nginx.nix ];
-  services.nginx.virtualHosts."cloud.graven.dev" = {
-    enableACME = true;
-    forceSSL = true;
-  };
-}