outcome results: add ratings graph

fixes CNVS-10612

test plan:
  * open outcome gradebook
  * click on a column label
  * verify that a pie chart is displayed in the popover,
      as in the mockup
  * verify that the chart matches the student outcomes data
  * verify that the popover handles outcomes with no results
  * verify that it works in Chrome

Change-Id: I30c0cb2ba3044d810cb020343ca70e448b038bd9
Reviewed-on: https://gerrit.instructure.com/28854
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>