gradebook: sort results to bottom

fixes CNVS-10714

test plan:
  * create an outcome with submissions from only some students
  * open gradebook
  * sort the column with incomplete submissions
  * verify that rows without submissions sort to the bottom
      in both assignment gradebook and outcome gradebook

Change-Id: I563b528a4620a38998d9e15a432ab246aa6854b6
Reviewed-on: https://gerrit.instructure.com/30236
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>