New accounts may be called "Deleted Editor #nnn"

When creating a new account, it is possible to choose the name "Deleted Editor #123456" or similar. This is not only confusing, it also prevents editor with ID 123456 from deleting their account, because the renaming to "Deleted Editor #123456" fails. Since the unique index is case insentitive, "deleted editor #nnn" and other variants should be forbidden, too.