Access Control: add migration 4 (#592).

Migration 4 deletes `dm4.accesscontrol.owner` and `dm4.accesscontrol.acl` properties from the DB.

Note: since DM 4.5 only workspaces have a `dm4.accesscontrol.owner` property. Other topics/associations have not.

The `dm4.accesscontrol.acl` property is obsolete.

See #592.