use polyfill for browsers not implemented Promise

lib/views/layout/layout.html

@@ -24,6 +24,7 @@
 
   <link rel="stylesheet" href="/css/crowi{% if env  == 'production' %}.min{% endif %}.css">
 
+  <script src="{{ assets('/js/polyfill.js') }}"></script>
   <script src="{{ assets('/js/bundled.js') }}"></script>
   <link href='//fonts.googleapis.com/css?family=Open+Sans:400,700' rel='stylesheet' type='text/css'>
 </head>

webpack.config.js

@@ -11,6 +11,7 @@ var config = {
     presentation: './resource/js/crowi-presentation.js',
     form:         './resource/js/crowi-form.js',
     admin:        './resource/js/crowi-admin.js',
+    polyfill:     'babel-polyfill',
   },
   output: {
     path: path.join(__dirname + "/public/js"),