webpack: Compress css in production mode

Adjust the WEBPACK_TEST file, as index.css is now not reliable (in
production mode it's .css.gz).

Closes #395

webpack.config.js

@@ -23,7 +23,7 @@ const plugins = [
 /* Only minimize when in production mode */
 if (production) {
     plugins.unshift(new CompressionPlugin({
-        test: /\.(js|html)$/,
+        test: /\.(js|html|css)$/,
         deleteOriginalAssets: true
     }));
 }