Introduce common JavaScript file (#2981)

* Create common chunk rather than vendor chunk

vendor chunk is a set of modules provided by external vendors, but now we
can have a chunk as a set of modules shared by multiple entry points,
which could be more efficent than having vendor chunk.

* Start rails-ujs in common.js

This is used by /settings/two_factor_authentication.

config/webpack/shared.js

@@ -40,8 +40,8 @@ module.exports = {
     new ExtractTextPlugin(env.NODE_ENV === 'production' ? '[name]-[hash].css' : '[name].css'),
     new ManifestPlugin({ fileName: paths.manifest, publicPath, writeToFileEmit: true }),
     new webpack.optimize.CommonsChunkPlugin({
-      name: 'vendor',
-      minChunks: ({ resource }) => /node_modules/.test(resource)
+      name: 'common',
+      minChunks: 2
     })
   ],