Initialize a default.nix for nix-build

As the previous commit mentions, I'm attempting to build and deploy this project
with `nix-shell` and `nix-build` instead of `cabal` and `stack`.

I'm in the Hamburg airport right now, and my internet connection isn't stable
enough to test this, so I'm committing it until I can more robustly test it.

website/sandbox/learnpianochords/src/server/default.nix

@@ -0,0 +1,27 @@
+let
+  pkgs = import /home/wpcarro/nixpkgs {};
+  ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: [
+    hpkgs.servant-server
+    hpkgs.aeson
+    hpkgs.wai-cors
+    hpkgs.warp
+    hpkgs.jwt
+    hpkgs.unordered-containers
+    hpkgs.base64
+    hpkgs.http-conduit
+    hpkgs.rio
+  ]);
+in pkgs.stdenv.mkDerivation {
+  name = "LearnPianoChords-server";
+  srcs = builtins.path {
+    path = ./.;
+    name = "LearnPianoChords-server-src";
+  };
+  buildPhase = ''
+    ${ghc} -O Main.hs \
+      -XOverloadedStrings \
+      -XNoImplicitPrelude \
+      -XRecordWildCards \
+      -XTypeApplications
+  '';
+}