Convert Beatmapset discussion post to typescript

[notbakaneko]

Now with more mobx and less data that doesn't exist.

The post.system checks are just there for typing checks, the class itself shouldn't getting any system posts but that's a typing fix for another time.

There's more changes to be done involving Editor that I'm not doing without #9062. Known existing issue where after making an edit and then navigating away and back, the unedited version shows.

• [ ] #9162
• [ ] #9039