feat: Runtime config support, take 2

Adds a couple of missing features for proper runtime configuration:

1. Favicon runtime configuration support via react-helmet

2. Placeholder values for APP_ID and MFE_CONFIG_API_URL in the sample
   .env files

src/index.jsx

@@ -2,11 +2,12 @@ import 'core-js/stable';
 import 'regenerator-runtime/runtime';
 
 import {
-  APP_INIT_ERROR, APP_READY, subscribe, initialize, mergeConfig,
+  APP_INIT_ERROR, APP_READY, subscribe, initialize, mergeConfig, getConfig,
 } from '@edx/frontend-platform';
 import { AppProvider, AuthenticatedPageRoute, ErrorPage } from '@edx/frontend-platform/react';
 import ReactDOM from 'react-dom';
 
+import { Helmet } from 'react-helmet';
 import { Switch } from 'react-router-dom';
 import messages from './i18n';
 
@@ -17,6 +18,9 @@ import PageContainer from './components/page-container/PageContainer';
 subscribe(APP_READY, () => {
   ReactDOM.render(
     <AppProvider>
+      <Helmet>
+        <link rel="shortcut icon" href={getConfig().FAVICON_URL} type="image/x-icon" />
+      </Helmet>
       <div className="pb-3 container">
         <Switch>
           <AuthenticatedPageRoute path="/courses/:courseId/bulk_email">