Use contenthash for ExtractTextWebpackPlugin (#5462)

[hash] is not documented.

config/webpack/shared.js

@@ -55,7 +55,7 @@ module.exports = {
         resource.request = resource.request.replace(/^history/, 'history/es');
       }
     ),
-    new ExtractTextPlugin(env.NODE_ENV === 'production' ? '[name]-[hash].css' : '[name].css'),
+    new ExtractTextPlugin(env.NODE_ENV === 'production' ? '[name]-[contenthash].css' : '[name].css'),
     new ManifestPlugin({
       publicPath: output.publicPath,
       writeToFileEmit: true,