Fixes #178: 'Event reached date' trigger fires daily on same participants

Description of the issue is in #178 (closed).

This reflects the change in !172 (merged) that assumes this trigger should only fire once per participant record.

Before !172 (merged), if the event date was changed, the rule could potentially fire a second time. After !172 (merged), it's implicit that every participant triggers the rule exactly once (unless a participant record is transferred to a new contact).

I can imagine scenarios where both the pre-!172 and post-!172 behavior are desirable when an event date changes, but a) this is an edge case, b) it's an undefined behavior, c) I think on the whole, the post-!172 behavior is safer.