ahmed: Factor configuration.nix into smaller submodules

I checked that this didn't result in any (non-trivial) changes like so:

$ vi hosts/ahmed/configuration.nix
$ nixos-rebuild build --flake .#ahmed
$ nix-diff /run/current-system ./result

1 files changed, 18 insertions, 0 deletions

diff --git a/hosts/ahmed/nginx/default.nix b/hosts/ahmed/nginx/default.nix
new file mode 100644
index 0000000..24dde85
--- /dev/null
+++ b/hosts/ahmed/nginx/default.nix
@@ -0,0 +1,18 @@
+# This module sets up an NGINX on this host.
+#
+# Different services' will register themselves with NGINX via
+# `services.nginx.virtualHosts`. They may also want to order themselves before
+# NGINX `systemd.services.*.{before,wantedBy}`.
+{config, ...}: {
+  # Virtual hosts.
+  services.nginx.enable = true;
+
+  # Configure ACME. This is used by various HTTP services through the NGINX virtual hosts.
+  security.acme = {
+    acceptTerms = true;
+    defaults.email = "linusvejlo+${config.networking.hostName}[email protected]";
+  };
+
+  # Allow HTTP connections.
+  networking.firewall.allowedTCPPorts = [80 443];
+}