Code style improvements in JavaScript (#13159)

* JS-linter: fix trailing comma's

* Configure eslinter to ignore this onchange error.

config/webpack/shared.js

@@ -31,7 +31,7 @@ module.exports = {
     Object.keys(themes).reduce((themePaths, name) => {
       themePaths[name] = resolve(join(settings.source_path, themes[name]));
       return themePaths;
-    }, {})
+    }, {}),
   ),
 
   output: {
@@ -73,7 +73,7 @@ module.exports = {
         // temporary fix for https://github.com/ReactTraining/react-router/issues/5576
         // to reduce bundle size
         resource.request = resource.request.replace(/^history/, 'history/es');
-      }
+      },
     ),
     new MiniCssExtractPlugin({
       filename: 'css/[name]-[contenthash:8].css',