gradebook: remember last visited tab

fixes CNVS-10632

test plan:
  * enable and open outcome gradebook
  * leave and re-enter gradebook
  * verify that the tab you last viewed opens immediately

Change-Id: I3d3e1cc3c53ef1f860ed2fe63f9c190372ecca08
Reviewed-on: https://gerrit.instructure.com/30783
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>