Use prettier URLs for vendor static assets

Get rid of the ugly `node_modules` paths in the downloaded JS resources.

Test Plan: `npm start`, then hit http://localhost:3000/ in the browser
and see the widgets list.

public/index.html

@@ -22,9 +22,9 @@
       }
     </script>
     <script src="http://localhost:3000/webpack-dev-server.js"></script>
-    <script src="node_modules/react/dist/react.min.js"></script>
-    <script src="node_modules/react/dist/react-dom.min.js"></script>
-    <script src="node_modules/react-relay/dist/relay.js" onerror="warnRelayMissing()"></script>
+    <script src="react/react.min.js"></script>
+    <script src="react/react-dom.min.js"></script>
+    <script src="relay/relay.js" onerror="warnRelayMissing()"></script>
     <script src="js/app.js"></script>
   </body>
 </html>

server.js

@@ -38,12 +38,12 @@ var app = new WebpackDevServer(compiler, {
 // Serve static resources
 app.use('/', express.static(path.resolve(__dirname, 'public')));
 app.use(
-  '/node_modules/react',
-  express.static(path.resolve(__dirname, 'node_modules/react'))
+  '/react',
+  express.static(path.resolve(__dirname, 'node_modules/react/dist'))
 );
 app.use(
-  '/node_modules/react-relay',
-  express.static(path.resolve(__dirname, 'node_modules/react-relay'))
+  '/relay',
+  express.static(path.resolve(__dirname, 'node_modules/react-relay/dist'))
 );
 app.listen(APP_PORT, () => {
   console.log(`App is now running on http://localhost:${APP_PORT}`);