Replace Prelude with RIO

I believe RIO stands for: "ReaderT <something-something> IO", which is a nod to
the top-level application data type:

```haskell
-- This is a simplification
newtype RIO env a = RIO { runRIO :: ReaderT env a () }
```

I read about RIO from an FP-Complete blog post a few months ago, and now I'm
excited to try it out for a real project. Bon voyage!

website/sandbox/learnpianochords/src/server/Spec.hs

@@ -1,6 +1,7 @@
 --------------------------------------------------------------------------------
 module Spec where
 --------------------------------------------------------------------------------
+import RIO
 import Test.Hspec
 import Utils
 import Web.JWT (numericDate, decode)
@@ -18,7 +19,7 @@ main = hspec $ do
     describe "validateJWT" $ do
       let validateJWT' = GoogleSignIn.validateJWT True
       it "returns a decode error when an incorrectly encoded JWT is used" $ do
-        validateJWT' (GoogleSignIn.EncodedJWT "rubbish") `shouldReturn` DecodeError
+        validateJWT' (GoogleSignIn.EncodedJWT "rubbish") `shouldReturn` CannotDecodeJWT
 
       it "returns validation error when the aud field doesn't match my client ID" $ do
         let auds = ["wrong-client-id"]