move most of the app into a src/app subdir

I think the index stuff was getting too easy to confuse for app code!

src/index.js

@@ -1,7 +1,7 @@
 import React from "react";
 import ReactDOM from "react-dom";
 import "./index.css";
-import App from "./App";
+import App from "./app/App";
 ReactDOM.render(
   <React.StrictMode>
     <App />