Refactor: Rename ParticipationRequest to Participation
Participations were initially added in !82 (merged) and renamed to ParticipationRequest in !170 (merged). The idea was that we would distinguish between a request for participation (recruitment) and the actual participation itself (execution). We ended up using the same model for both, so the name change turned out to be for nothing.
This looks scary because it touches a lot of files, but it is actually not that complicated. Django's RenameModel migration operation takes care of all permissions and references.