Add mechanism to clear all absences of person in given time span

needed for AlekSIS/official/AlekSIS-App-Alsijil#349

changed milestone to /AlekSIS%"[NLnet #6 (closed)] Fix bugs/missing small features from pilot schools"

added part::backend type::feature labels

assigned to @yuha

created branch 39-add-mechanism-to-clear-all-absences-of-person-in-given-time-span to address this issue

mentioned in merge request !66 (merged)

closed with commit 636ae38b

mentioned in commit 636ae38b