outcome results report: add outcome details popup

fixes CNVS-10294

test plan:
  * open gradebook in a course with outcome gradebook enabled
  * click on an outcome column label
  * verify that a popup is displayed
  * verify that it includes title and description
  * verify that it matches the design spec

Change-Id: Ia4a6a3f3ddd4e3445e468f7ae0400f1f798ac0a8
Reviewed-on: https://gerrit.instructure.com/28636
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>