Register absences and excuses centrally

Provide a way to register known absences of students centrally, also for multiple days and lesson periods.

How to:

• Add a new view in Alsijil, accessible to supraadmins only
• Give the view fields for start date, end date (both defaulting to today), person, absent and excused (both defaulting to True), starting lesson (defaulting to 0)
• On saving. call mark_absent for every day in the selected date range on the selected person. If it spans multiple days, disregard the starting lesson for all but the first day!