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/sources/default.nix

@@ -1,11 +0,0 @@
-let
-  sources = import ./nix/sources.nix;
-
-  # just use standard pkgs from sources
-  # so that we have our applyPattches function
-  pkgs = import sources.nixpkgs {};
-
-in {
-  nixus = sources.nixus;
-} // sources
-