Add API fields for review request counters and add new query args.

The Review Request resource now provides fields for `ship_it_count`,
`issue_open_count`, `issue_dropped_count`, and `issue_resolved_count`.

Callers can filter these for exact matches by passing, say,
`?ship-it-count=<value>`. It can also filter through ranges by
appending `_lt`, `_lte`, `_gt`, or `_gte` on the query parameter.
The results will be filtered accordingly.