Resolve "there should be a way to add an appointment with external person (neither a subject nor control)"

Closes #28 (closed)

merged

mentioned in commit 270273da

migration script should also include this code to remove workaround:

update web_appointment  set visit_id = null where visit_id in (select id from web_visit where subject_id in (select id from web_subject where screening_number='---'));
delete from web_visit where subject_id in (select id from web_subject where screening_number='---');
delete from web_subject where screening_number='---';