Serve the new website at dhall-lang.org (#319)

Fixes #320

This changes `https://dhall-lang.org` to serve the new website for the
language instead of redirecting to the GitHub project

nixops/dhall-haskell.json

@@ -0,0 +1,7 @@
+{
+  "url": "https://github.com/dhall-lang/dhall-haskell.git",
+  "rev": "21d70b338354f4fb9fce43b6fa08170dc9b30847",
+  "date": "2018-12-14T18:50:21-08:00",
+  "sha256": "0mbv0yj3pyjlmdm6bwf0npd833ivgj39p4g4sq6dknpyxm7073gl",
+  "fetchSubmodules": false
+}

nixops/logical.nix

@@ -117,28 +117,48 @@
 
         recommendedTlsSettings = true;
 
-        virtualHosts."dhall-lang.org" = {
-          addSSL = true;
+        virtualHosts."dhall-lang.org" =
+          let
+            json = builtins.fromJSON (builtins.readFile ./dhall-haskell.json);
 
-          default = true;
+            dhall-haskell =
+              pkgs.fetchFromGitHub {
+                owner = "dhall-lang";
 
-          enableACME = true;
+                repo = "dhall-haskell";
 
-          locations."/".extraConfig = ''
-            rewrite ^.*$ https://github.com/dhall-lang/dhall-lang/blob/master/README.md redirect;
-          '';
+                inherit (json) rev sha256;
+              };
+
+            dhall-haskell-derivations =
+              import "${dhall-haskell}/default.nix";
+
+            inherit (dhall-haskell-derivations) try-dhall;
+
+          in
+            { forceSSL = true;
+
+              default = true;
+
+              enableACME = true;
+
+              locations."/" = {
+                index = "index.html";
+
+                root = "${try-dhall}";
+              };
         };
 
         virtualHosts."cache.dhall-lang.org" = {
-          addSSL = true;
+          forceSSL = true;
 
           enableACME = true;
 
           locations."/".proxyPass = "http://127.0.0.1:5000";
         };
 
         virtualHosts."hydra.dhall-lang.org" = {
-          addSSL = true;
+          forceSSL = true;
 
           extraConfig = ''
             proxy_set_header Host $host;
@@ -156,7 +176,7 @@
         };
 
         virtualHosts."prelude.dhall-lang.org" = {
-          addSSL = true;
+          forceSSL = true;
 
           enableACME = true;
 
@@ -235,6 +255,16 @@
         wantedBy = [ "multi-user.target" ];
       };
 
+      kick-hydra-evaluator = {
+        script = ''
+          ${pkgs.systemd}/bin/systemctl restart hydra-evaluator
+        '';
+
+        startAt = "*:0/5";
+
+        wantedBy = [ "multi-user.target" ];
+      };
+
       nix-serve-keys = {
         script =
           let