student learning mastery: fix outcome group hierarchy

test plan:
  * create an outcome group hierarchy large enough that the root
      outcome group is not last when requesting
      /api/v1/courses/X/outcome_groups
  * open the student learning mastery report
  * verify that the hierarchy is presented correctly

Change-Id: Id71e8c292517e78012cc638edaffa4bbca564a42
Reviewed-on: https://gerrit.instructure.com/36538
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>