Implement hook_user_cancel

aaron.ferris → made their first commit to this issue’s fork.

Made an initial start on this, we will have to do it via hook_user_cancel for the user_cancel_reassign cancellation method, but also a hook_ENTITY_TYPE_delete for the user_cancel_delete method, because as per https://api.drupal.org/api/drupal/core%21modules%21user%21user.api.php/f...

"This hook is NOT invoked for the 'user_cancel_delete' account cancellation method. To react to that method, implement hook_ENTITY_TYPE_predelete() or hook_ENTITY_TYPE_delete() for user entities instead."

Need to test the other methods, which I will do over the next couple of days.