porthos: split into modules

I have separated the modules into host-specific settings, and generic
settings that ought to be shared by every host.

I only have the 'porthos' host for now, but intend to also add my laptop
'aramis' at some point to this repository.

porthos.nix

@@ -0,0 +1,23 @@
+# Porthos self-hosted server
+{ ... }:
+
+{
+  imports = [
+    # Include generic settings
+    ./modules
+    # Include porthos-specific modules
+    ./porthos
+    # Include my secrets
+    ./secrets
+    # Include my services
+    ./services
+  ];
+
+  # This value determines the NixOS release from which the default
+  # settings for stateful data, like file locations and database versions
+  # on your system were taken. It‘s perfectly fine and recommended to leave
+  # this value at the release version of the first install of this system.
+  # Before changing this value read the documentation for this option
+  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
+  system.stateVersion = "20.09"; # Did you read the comment?
+}