Add absence management to course book student dialog

Managing a student's absence status linked to a documentation should take place in the same dialog used for managing personal notes on per-student base. Additionally, the student list of a lesson has to show the absence status of a student for the respective documentation (see mock-up); and the column containing student-related information in the course book table (#259 (closed)) should be enriched with aggregated information about the class' absence status (e.g. how many students were present; see mock-up).