Fix handling of new absences outside of existing ones

aleksis/apps/kolego/models/absence.py

@@ -220,6 +220,7 @@ class Absence(FreeBusy):
                         event_within.delete()
                     elif (
                         new_datetime_start > event_within_datetime_start
+                        and new_datetime_start < event_within_datetime_end
                         and new_datetime_end >= event_within_datetime_end
                     ):
                         # Cut end of existing event
@@ -228,6 +229,7 @@ class Absence(FreeBusy):
                     elif (
                         new_datetime_start <= event_within_datetime_start
                         and new_datetime_end < event_within_datetime_end
+                        and new_datetime_end > event_within_datetime_start
                     ):
                         # Cut start of existing event
                         event_within.datetime_start = new_datetime_end