Upgrade to active_model_serializers 0.8.1.  #4175.

Two reasons:

1. Keeping up.
2. The 0.8.x series changed the way serializer roots are set, and I'd
   like to be able to use a syntax that's been changed due to (what is
   presumably) feedback from actual usage.
Write participant first and last names in event report.  #4175.

Also fix an incorrect assumption that event disposition will always be
present.
Return the participant's public ID, not its internal ID.
Maintain EventReportRowSerializer as an implementation detail.  #4175.
Merge pull request #200 from NUBIC/task-4175-participant-names

Return participant names in GET /api/v1/events, and related small fixes