Move Haskell-related shell.nix code into its own shell.nix

I'm getting tired of:

```shell
$ cd <project-root>
$ nix-shell
$ cd src/server
$ ghci Main.hs
```

Instead:

```shell
$ cd <project-root>/src/server
$ ghci Main.hs
```

2 files changed, 19 insertions, 11 deletions

diff --git a/website/sandbox/learnpianochords/shell.nix b/website/sandbox/learnpianochords/shell.nix
index 9402ad5674d2..6cc05d099177 100644
--- a/website/sandbox/learnpianochords/shell.nix
+++ b/website/sandbox/learnpianochords/shell.nix
@@ -5,16 +5,5 @@ in pkgs.mkShell {
     elmPackages.elm
     elmPackages.elm-format
     elmPackages.elm-live
-    (haskellPackages.ghcWithPackages (hpkgs: [
-      hpkgs.hspec
-      hpkgs.servant-server
-      hpkgs.aeson
-      hpkgs.wai-cors
-      hpkgs.warp
-      hpkgs.jwt
-      hpkgs.unordered-containers
-      hpkgs.base64
-      hpkgs.http-conduit
-    ]))
   ];
 }
diff --git a/website/sandbox/learnpianochords/src/server/shell.nix b/website/sandbox/learnpianochords/src/server/shell.nix
new file mode 100644
index 000000000000..6cb5e19e99fa
--- /dev/null
+++ b/website/sandbox/learnpianochords/src/server/shell.nix
@@ -0,0 +1,19 @@
+let
+  pkgs = import /home/wpcarro/nixpkgs {};
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    (haskellPackages.ghcWithPackages (hpkgs: [
+      hpkgs.hspec
+      hpkgs.servant-server
+      hpkgs.aeson
+      hpkgs.wai-cors
+      hpkgs.warp
+      hpkgs.jwt
+      hpkgs.unordered-containers
+      hpkgs.base64
+      hpkgs.http-conduit
+      hpkgs.rio
+      hpkgs.envy
+    ]))
+  ];
+}