outcome results API: include result count on rollups

fixes CNVS-13365

test plan:
  * open /api/v1/courses/1/outcome_rollups
  * verify that each score includes a count
  * verify that counts match the number of results for that
      combination of outcome and student

Change-Id: I0cbbab7d2c1ebb6574e090046c52b7a4159bb9b9
Reviewed-on: https://gerrit.instructure.com/35708
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>