Contrast Ratio Fixed in Gradebook

fixes CNVS-12669

This fixes the contrast ratio issues we were having on normal and
high contrast UI.

Testing Process:

- Make sure you have letter grading enabled for assignments
- Make sure you have some students that have had assignments
graded
- As a teacher or admin, go to Grades
- The letter grade that shows in the "Total" column should be passing
contrast in Webaim for Regular and High Contrast now
Example: http://cl.ly/image/350X350X0623
- Click on a student
- The letter grade that shows up in the "Score" column should also
be passing Webaim for contrast in both regular and High contrast
settings
Example: http://cl.ly/image/3F2e352v3x1O

Change-Id: Ibb92571df760add588e998e82bd0455eca322a77
Reviewed-on: https://gerrit.instructure.com/41066
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Colleen Palmer <colleen@instructure.com>