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/Fixtures.hs

@@ -1,6 +1,7 @@
 --------------------------------------------------------------------------------
 module Fixtures where
 --------------------------------------------------------------------------------
+import RIO
 import Web.JWT
 import Utils