Support echo server to test POST /verify

TL;DR:
- Add common dependencies like Servant, Aeson, Warp, Cors
- Define a POST /verify endpoint for our client to hit
- POST to /verify client-side onSignIn

website/sandbox/learnpianochords/shell.nix

@@ -5,8 +5,12 @@ in pkgs.mkShell {
     elmPackages.elm
     elmPackages.elm-format
     elmPackages.elm-live
-    (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
-      hspec
+    (haskellPackages.ghcWithPackages (hpkgs: [
+      hpkgs.hspec
+      hpkgs.servant-server
+      hpkgs.aeson
+      hpkgs.wai-cors
+      hpkgs.warp
     ]))
   ];
 }