Before, on the backend if you check the `opt_out` checkbox and saving on a `mailing.contact`, the entire row (`mailing.contact.subscription`) would be unlinked. Combined with `mass_mailing_list_dynamic` this means the contact would just be re-subscribed over and over.
After, check `opt_out` will fill `unsubscription_date` and the row will stay. Tests in `mass_mailing_list_dynamic` pass.
