Introduce "substitution to-do-list" generated from teachers absences

Person x is absent from y to z → Define absences (this is already planned in Kolego (https://edugit.org/AlekSIS/onboarding/aleksis-app-kolego/-/issues/1)

• Overview with all days; then a to-do list per day with hours to cover (generated on manual click of a button that syncs absences to this list)