Add for_participant manager

There is already a for_teacher & a for_person manager. But for_participant is so far missing.

changed milestone to /AlekSIS%"[NLnet #3 (closed)] Implement new course book"

assigned to @permcu

created branch 243-add-for_participant-manager to address this issue

mentioned in merge request !348 (merged)

mentioned in commit 6762c3f0

closed with merge request !348 (merged)