outcomes gradebook: add rating statistics

fixes CNVS-10595

test plan:
  * open outcomes gradebook
  * click on an outcome column label
  * verify that the popover includes rating percentages
  * verify that the percentages match your outcome configuration
      and the student results

Change-Id: I1a295e405200b7c981f4b0a7dc8f75b714dd5000
Reviewed-on: https://gerrit.instructure.com/28828
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>