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.

stable/deploy/default.nix

@@ -0,0 +1,24 @@
+let
+  sources = import ../config/sources;
+in import "${sources.nixus}" {} ({ config, ... }: {
+
+  defaults = { name, ... }: {
+    configuration = { lib, ... }: {
+      networking.hostName = lib.mkDefault name;
+    };
+
+    # use our nixpkgs from niv
+    nixpkgs = sources.nixpkgs;
+  };
+
+  nodes = {
+    mail = { lib, config, ... }: {
+      host = "emelie@mail.graven.dev";
+      configuration = ../config/hosts/mail/configuration.nix;
+      switchTimeout = 300;
+      successTimeout = 300;
+      #ignoreFailingSystemdUnits = true;
+    };
+  };
+})
+