added feiertage to database + endpoint to insert every year

Backend/models/absence.go

@@ -45,6 +45,10 @@ func (a *Absence) Date() time.Time {
 	return a.Day.Truncate(24 * time.Hour)
 }
 
+func (a *Absence) Type() DayType {
+	return DayTypeAbsence
+}
+
 func (a *Absence) IsMultiDay() bool {
 	return !a.DateFrom.Equal(a.DateTo)
 }