gradebook: better sorting for total columns

fixes CNVS-9300

Test plan:
  * make a course with 2 assignments:
    * one has 10 points possible
    * one has 100 points possible
  * give a student 10/10 on the first assignment (no score on the
    second)
  * give another student 20/100 on the second assignment
  * sort by total grade
  * it should sort by percent
  * switch the total grade to display as points
  * it should sort by points

Change-Id: Ie0651fcefb3c0880a017c438b40aa169158f34ba
Reviewed-on: https://gerrit.instructure.com/36664
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>