webpack-config: export NODE_ENV=production when not in debug mode

{{cookiecutter.app_name}}/webpack.config.js

@@ -59,5 +59,10 @@ module.exports = {
     }),
   ].concat(debug ? [] : [
     // production webpack plugins go here
+    new webpack.DefinePlugin({
+      'process.env': {
+        'NODE_ENV': JSON.stringify('production')
+      }
+    }),
   ])
 };