fixed problem with the duration of auto created follow up visits. relative date was in days instead of months

- Add field default_delta_time_for_control_follow_up to study
- Add field default_delta_time_for_follow_up_units to study
- Add field default_delta_time_for_patient_follow_up to study
- Alter field default_visit_duration_in_months on study
- Alter field default_voucher_expiration_in_months on study

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

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

added order in daily planning based on officeAvailability which considers holidays, extra_availabilities, general appointments from Appointment, and subject assesments from AppointmentTypeLink

removed appointments reference from model. Filter out extra availabilities for the method is_on_leave

Fixed problem in commit 47b22f88. Tests didn't fail on my machine. God save continuous integration. Tests passed

Changed function so the reminder is sorted as well. If no filter is provided then the reminder is all the screening number and it will be sorted with the provided order. Changed tests and the test screening numbers.

Issue #207 . Added custom sorting for screening field in study subject based on the filter introduced in the column. Multivalues are re-sorted

Issue #239 Removed get_details method and changed extend_context method to use Worker.get_by_user. Enhancement: now the superuser also shows a name in the upper right corner or its username if no full_name is available.