fix speedgrader multiple submission bug

refs CNVS-13920, refs CNVS-13447

Test plan:
  * submit multiple crocodocable submissions to an assignment for one
    student
  * submit a single crocodocable submission for another student
  * try grading each version of the submissions in speedgrader
  * after grading, you should remain on the same submission version
    (but switching versions should still work)

Change-Id: Idb4244b0ba0f92550a8dbd761a013a67fca8d33a
Reviewed-on: https://gerrit.instructure.com/37711
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>