must approve users is set and the user is rejected, we delete
User object however we keep ReviewableUser. It is good for audit
purpose however we should disassociate target_id because another
user may register with the same id.
In addition, I added a migration to fix data
The issue was mentioned here: https://meta.discourse.org/t/deleted-user-info-shows-up-in-new-user-approval/161720