Adding webpack code splitting (#3377)

webapp/components/admin_console/admin_sidebar_section.jsx

@@ -3,7 +3,7 @@
 
 import React from 'react';
 
-import {Link} from 'react-router';
+import {Link} from 'react-router/es6';
 
 export default class AdminSidebarSection extends React.Component {
     static get propTypes() {