Set Attendance Excusal

Record that one student's absence from one class was excused, and why.

Does NOT change attendance. The cell still says the student was not in the call, because they were not; this sits alongside it. Rewriting presence would destroy the only honest record of who was actually there, which is the thing the sheet exists to be.

Authorised for the staff of the section that class belongs to, plus owners and admins — the same door _require_group_staff guards everywhere else, so a teacher cannot excuse an absence in someone else's section.