outcome gradebook: use open arrow icon

fixes CNVS-11118

test plan:
  * enable and open outcome gradebook
  * verify that a hollow arrow icon is used for the sidebar toggle
  * toggle the sidebar
  * verify that the appropriate icons are used

Change-Id: Idee2079a03a1eea4141a5d132045acc7124b5a3c
Reviewed-on: https://gerrit.instructure.com/30710
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>