Issue #278 Avoid deleting availabilities from the daily planning and add more control for undefined values

Issue #276 changed check_nd_number_regex to receive a study and to filter out null or blank nd_numbers in the studySubjects

Issue #266 Fixes problem when no voucher is checked to be printed. Rises a tooltip. But, even if the call was produced and no template was found in generate_for_vouchers, it would be redirected to the previous page (subject edit) or, if no previous page is found, to the subject list

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