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

Feature/dummy data pdp

See merge request NCER-PD/scheduling-system!172

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

Bug/hide vouchers if not set in study

Closes #246

See merge request NCER-PD/scheduling-system!174

improved text message in button to go back

Closes #244

See merge request NCER-PD/scheduling-system!175

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

Feature/export selected columns

See merge request NCER-PD/scheduling-system!173

Feature/daily availability

See merge request NCER-PD/scheduling-system!171

Merge branch 'feature/dummy_data_pdp' of ssh://git-r3lab-server.uni.lu:8022/NCER-PD/scheduling-system into feature/dummy_data_pdp