Add ui subfolder, bootstrapped a "hello-world" React-Admin app, changed Makefile to start both apps in dev mode

ui/src/App.js

@@ -0,0 +1,12 @@
+// in src/App.js
+import React from 'react'
+import { Admin, ListGuesser, Resource } from 'react-admin'
+import jsonServerProvider from 'ra-data-json-server'
+
+const dataProvider = jsonServerProvider('http://jsonplaceholder.typicode.com')
+const App = () => (
+  <Admin dataProvider={dataProvider}>
+    <Resource name="users" list={ListGuesser} />
+  </Admin>
+)
+export default App