- Rename the audit actor columns user_type/user_id to actor_type/actor_id
- Add owner_organization_id and owner_user_id to the audits table as
foreign keys with cascade on delete, so that the audits of an
organization or user are deleted together with it. The indexes and
foreign keys are created without blocking writes on the large table.
- Fill the owner columns for new audits via CustomAuditable. Organizations
and users no longer record their own deletion audit, since it would
reference the already deleted owner.
