Set `CreatedModifiedMixin`'s user relations's `on_delete` to `PROTECT`
Protects us from accidentally deleting content by deleting a user which has created or modified it.
If we anyways want to delete a user that has edited something, we should manually set created_by
or modified_by
to Trine Taktlaus, or something similar.
Edited by Sigurd Tullander