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

Avoid possibility to change the worker when adding holidays or extra availabilities. Changed the header

Fixes issue #222 in which the voucher partner and health partner shown the option to select role. This option is now hidden.