warn teachers when changing grade display

fixes CNVS-11250

test plan:
  - as a teacher with unweighted assignment groups
  go to GB2 and toggle between points/percent
  - a modal should appear that warns you about this action
    - the text should make sense
    - it should say "a percent" or "points" depending on
      your current settings
    - clicking cancel or the x should close the dialog
    - clicking continue should close the dialog and change the totals
      - when you reload the page, totals should remain changed
    - clicking the checkbox and then continue should do the same
      - however the next time you click the dropdown, the modal should
        not appear, even after reload
        (other courses/browsers will still show this user the modal)
    - as a TA or another teacher in the same class, the modal should
      still appear (until you click the checkbox)

Change-Id: I465bd7b9d770077931a769efb7904c1349920a3c
Reviewed-on: https://gerrit.instructure.com/33356
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>