Make Gradebook 2 high-contrast UI more accessible

Fixes: CNVS-15557

Visual summary of changes:
http://cl.ly/image/181x071u3e2B/CNVS-15557.png

Multiple small UI tweaks to increase foreground-
background contrast in Gradebook 2.

IMPORTANT: This ticket is only for the high-
contrast UI. To enable the high-contrast UI, go
to Settings then scroll down and make sure
"Use High Contrast Styles" is set to ON.

Test plan:

- Enable high contrast UI in Canvas (see above)
- Go through each issue Nathan entered into the
  JIRA and compare with the updated UI to make
  sure it's fixed.
- The webAIM color contrast tester can be found
  here: http://webaim.org/resources/contrastchecker/

Change-Id: I1051691ad6be2911e67c5bb7c637d96c30a38b80
Reviewed-on: https://gerrit.instructure.com/41711
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>