move gradebook section filter to header

fixes CNVS-10293

test plan:
  * enable outcome gradebook feature, navigate to
    gradebook2 as a teacher in a course with multiple
    sections;
  * verify that the section filter is displayed in the
    gradebook header to the right of the navigation pills;
  * verify that the filter works in both the outcome
    gradebook and the normal gradebook;
  * view a course with no sections and verify that the
    filter is not displayed;
  * disable the outcome gradebook feature and verify that
    the filter displays inline with the gradebook in its
    old position.

Change-Id: I2576fa50e33c5934843e509924da0e89cbadee3d
Reviewed-on: https://gerrit.instructure.com/28418
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>