Only allow one absence at any point of time

There should only be one absence per person at any given time. If an absence overlapping another absence is added, the older absences should be cut in order to give room for the new absence.

added part::backend label

assigned to @yuha

mentioned in issue AlekSIS/official/AlekSIS-App-Alsijil#327

created branch 32-only-allow-one-absence-at-any-point-of-time to address this issue

mentioned in merge request !52 (merged)

mentioned in commit 7bac57ff

closed with merge request !52 (merged)