Start reworking the server for nix flakes

This removes all existing services as well, in preparation of moving
them to `podman`. These are easier to update to
virtualisation.oci-containers while retaining the "networks" through
pods.

configuration/default.nix

@@ -0,0 +1,57 @@
+{ config, pkgs, ... }:
+
+{
+  imports = [ ];
+
+  nix = {
+    package = pkgs.nixFlakes;
+    extraOptions = ''
+      experimental-features = nix-command flakes
+    '';
+  };
+
+  networking = {
+    hostName = "tlaternet";
+
+    usePredictableInterfaceNames = false;
+    useDHCP = false;
+    interfaces.eth0.useDHCP = true;
+
+    firewall.allowedTCPPorts = [ 80 443 2222 2221 25565 ];
+  };
+
+  time.timeZone = "Europe/London";
+
+  users.users.tlater = {
+    isNormalUser = true;
+    extraGroups = [ "wheel" ];
+    openssh.authorizedKeys.keyFiles = [ ../keys/tlater.pub ];
+  };
+
+  services.openssh = {
+    enable = true;
+    allowSFTP = false;
+    passwordAuthentication = false;
+    permitRootLogin = "no";
+    ports = [ 2222 ];
+    startWhenNeeded = true;
+  };
+
+  services.nginx = {
+    enable = true;
+    recommendedTlsSettings = true;
+    recommendedOptimisation = true;
+    recommendedGzipSettings = true;
+    recommendedProxySettings = true;
+    clientMaxBodySize = "10G";
+  };
+
+  security.acme = {
+    email = "tm@tlater.net";
+    acceptTerms = true;
+  };
+
+  virtualisation.oci-containers.backend = "podman";
+
+  system.stateVersion = "20.09";
+}