Document new report flow.  #4161.
EventReport now requires a date range.  #4161.
Rearrange specs from least to most specific.  #4161.
Remove report code paths that don't involve a date range.  #4161.

As part of this work, trimmed out unnecessary alternative branches and
unused VCR-recorded requests.
Merge pull request #198 from NUBIC/task-4161-dates-first

Require a date range for GET /api/v1/events