add mean/median/mode dropdown to outcome gradebook

fixes CNVS-10291

test plan:
  * in a course with outcome gradebook enabled, navigate
    to it as a teacher;
  * verify that "course average" dropdown is present in
    the averages header row;
  * verify that you can change the dropdown value and
    that it updates the header values;
  * verify that the mean/median/mode are correct.

Change-Id: I98ceaa342f27988cc349323d50360c31e2e563bd
Reviewed-on: https://gerrit.instructure.com/28457
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>