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

added checks for forms
added check function in Study and StudySubject to avoid repeating a regex inline
removed previous checks in which the regex was rewritten
now the used regex is the one defined in the given study
added tests and migrations
changed functions in tests/functions to adapt the tests

Daily Planning: added tooltip with flags of the languages spoken by subjects and workers. Moved code to the resourceRender method