Better handling of user deleting

When deleting a user it appears that posts and topics are being left behind. Also if a user that has existing approved contributions is deleted we need to evaluate if that's ok. Can the contribution remain without an associated user? If so then maybe nothing needs to be done, otherwise we'll need to change to anonymous and ensure that it can be transferred to someone else. Contributions without approved revisions can just be deleted.

2 relevant core events can be used for this:

• https://wiki.phpbb.com/Event_List#core.delete_user_after
• https://wiki.phpbb.com/Event_List#core.delete_user_before