Fixes most of #263 : nd number should have PDP prefix; screening numbers should be filled in (using P-) ; location prefix should be defined: P (CHL), F (Flying team)

added signals to Subject and StudySubjet. These signals fire after an instance is saved and check if the subject is excluded, resigned or dead and then call the methods that finish all the appointments and visits. It is safer than doing it on the forms or other parts of the code. Issue #203

Issue #259 : added test to send mail with general appointment and appointment types: test_kit_request_send_mail_with_general_appointment

moved function is_valid_social_security_number to utils so it can be used in other python files such as import_file.py