handle speed grader section change bugs

fixes CNVS-1648

test plan:
  - go to speedgrader with multiple sections
    and submissions for multiple students
  - make sure that the first student in one of the
    sections has submitted an assignment with an online
    submission multiple times and is not in the other section
  - toggle between sections
    - in the upper right corner the submission dropdown
      should not have ??? in it
  - mess around with changing sections in speed grader and
    make sure you cant get ??? to show up
  - reloading the page should keep the same student

Change-Id: I3515c2b1d902cb48da3096c2e8f1fdf47b3efc39
Reviewed-on: https://gerrit.instructure.com/31028
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>