Self-host comments in Jekyll, powered by Firebase real-time database

It's convenient to set up a comment system in Jekyll site with external social comment systems like Disqus or Duoshuo (多说). However, as you all know, Disqus was blocked in China and Duoshuo is going to shutdown. It's the time to rethink about the comment system (although I didn't get too many comments →_→), simple and controllable. And it becomes true with Firebase database.