Refactor the create new post action

[nigini]

The @router.post("/actions/new", response_model=None) (link) is one of the foundational actions in this software.

Currently, it packs too much on it, including:

• processing and creating different objects based on post type
• uploading attached files (should be a separate action)
• send "updates" to boxes (currently only creates...?)
• redirects user

I believe that at least file processing should be separated to facilitate functionalities like this one. Moreover, I also think that "updating" the AP boxes should be separated, which would make sense if we go in this direction.