message: add chaching backend (see cf worker)
diff: <%= htmlWebpackPlugin.options.firebugScript %>
  <script>
  var isWidgetBuild = '<%= htmlWebpackPlugin.options.isWidget %>';
  + try {
  + JSON.parse("<!-- BACK_SO -->");
  + } catch (e) {
  + console.log("warning: caching backend is down")
  + }
  </script>
  <style>
  