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.
* Exclude opt_out.
Now opted-out records will not be counted in the "Mailing lists" smart button
in the partner form.
* Avoid duplicate error.
By indicating the exact `partner_id` and ensuring no contacts associated to it are found, you avoid possible duplication errors when several partners share the same name or email.
