When deleting a user, author replacement should not occur after user deletion

Deleting a user before doing a cleanup of the pages on which he was the author isn't fine : in case the user that you are deleting has created 10k pages, the process will take ages.

During this migration process, all the pages that used to rely on the rights of the user being deleted will not function properly until they are migrated. In practice, if the user installed the wiki you're in, the wiki becomes pretty much unusable.