- It's unnecessary to save images in server, getting them directly from the umooc server is a better idea.
- As the umooc's support of emoji is just so bad, emojis are now converted into '[emoji]'
- The session needs to init for just one time, so prepare lines are separated into a individual function instead of running ever time before getting topic list
- Considering the existence of images, reply contents is a dict list (e.g. `{'type': 'text', 'content': 'foo'}` `{'type': 'img', 'img_id': 'bar'}`