Add absence overview page

The absence overview page (see mock-up) is a table containing students' absences of any kind, filtered by group/name/time span, thus allowing for efficient changing of their absence status (e.g. from non-excused absence to excused absence). This includes allowing bulk editing. It, too, is infinitely scrollable and should load its data dynamically. Additionally, links from the course book table to this page and vice versa have to be added.