outcome gradebook: display outcome path in column popover

fixes CNVS-10628

test plan:
  * create an outcome group with an outcome in it
  * enable and open outcome gradebook
  * hover over the column header for your nested outcome
  * verify that its path is displayed as the popover title

Change-Id: I40fcfe1ed7355c125d5e7c6791e7f0225cd539a8
Reviewed-on: https://gerrit.instructure.com/31785
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>