Introduction action: prevent double trigger
Introduction action: prevent double trigger

This fix changes logic for adding new instructors to the event (event
can be already fulfilling the check for this action).

Additionally, the action is now being saved on the event, instead of
the task, which should also contribute to the bug fix.

pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,pbanaszkiewicz/amy,swcarpentry/amy,swcarpentry/amy