porthos: move files into 'machines' directory

But keep 'porthos.nix' at the root of the repository. I feel like it is
cleaner to keep device specific files at the root.

machines/porthos/users.nix

@@ -0,0 +1,15 @@
+# User setup
+{ config, ... }:
+let
+  my = config.my;
+in
+{
+  users.users.blog = {
+    description = "Blog Publisher";
+    isNormalUser = true;
+    group = "nginx";
+    createHome = false; # Messes with permissions
+    home = "/var/www/";
+    openssh.authorizedKeys.keys = [ my.secrets.drone.ssh.publicKey ];
+  };
+}