Story | Implement delete_user CP API

[maxyu1115]

Implement delete_user CP API.

Simple as it sounds, this is not a straightforward implementation. Deleting a user will involve deleting all corpuses underneath them. It should also fail when they still have users underneath them.

We need to implement some sort task queue mechanism, where we delete the corpuses in the background. Also be wary of situations where a deletion is halted midway, we need to be able to restart the deletion and clean things up