allow displaing total grade as points for students

fixes CNVS-9874

when teacher choses to show total grade as points in GB2, a setting is saved
student grade summary page shows the total grade in same format
if assignment groups are weighted, grade is displayed as a percentage again

test plan:

  - new addition:
    - go to a course where GB2 is displaying totals as points BUT has no DB setting about show_point_totals
      (ask mike if you need help getting to this state)
    - as a student look at the grade summary page, total should be a percent
    - as a teacher in that class, go to GB2 and wait for 5 seconds
    - as that same student, go back to grade summary, total should be points now

  - as a teacher, change total grade to show as points in GB2
    - when you return to the GB, even from a new browser, this should be the same
    - in GB, tooltip should show points if cell shows percent, and visa vera
    - as a student in that class, you should see your total grade as points
      - grade tooltip should show points if cell shows percent, and visa vera
  - switch to weighted assignment groups
    - total grade as a teacher and student should switch back to percent
  - switch back to non-weighted
    - total grade should stay a percent until explicitly changed back to points

  - GB2 weighting and points/percent switching should have no unintended effects on the GB
  - student grade summary page should display everything in a consistent manner

Change-Id: Id0c4f496576c226eb7000d6684a37faf0b439359
Reviewed-on: https://gerrit.instructure.com/28780
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>