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

It is currently infeasible to support `GET /api/v1/events` calls that aren't bounded by a date range.  This PR contains

1. a modification to `Reports::EventReport` that enforces that restriction, and
2. some simplifications that result from that restriction.

See https://code.bioinformatics.northwestern.edu/issues/issues/show/4161 for more information.