Add Event#scheduled_activities=.  #3662.

I wanted to avoid this, but as it turns out, it's _really_ convenient
for event report generation, where we have two potential sources of
scheduled activities.  (The difference is because I want to narrow down
the result set as fast as possible.)
Factor out event-scheduled activity linkage context.  #3662.
.with_psc_data should eager-load PscParticipant requirements.  #3662.
If an event is missing a person or participant, don't load SAs.  #3662.

The schedules endpoint requires a person ID to retrieve a schedule for a
person, and in this case, that person ID is coming from the event's
Participant.
Event#{scheduled_date, data_collectors}.  #3662.
Expand the event-PSC linkage context.  #3662.

I want more events (hence, more activities) to better exercise the
Reports::EventReport code.
Reports::EventReport.  #3662.
GET /api/v1/events.  #3662.
Reports::EventReport serialization.  #3662.

Also update ncs_navigator_schema because the previous commit didn't
express our property expectations correctly.