srgb letter grades calculated and displayed properly

fixes: CNVS-12671

test plan:
  - set a course grading scheme
  - have weighted assignment groups
  - grade students so they have scores for each assignment group
  - srgb should display the letter grades for assignment groups & total properly
  - gb2 should show total letter grade properly as well

  - courses without grading standards shouldn't display
    letter grades for total in srgb

Change-Id: I8acafa9b7afbf62167d831f369bf87e582a9786f
Reviewed-on: https://gerrit.instructure.com/34019
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>