Color contrast - assignments page

Fixes: CNVS-15577

Multiple small UI tweaks to increase foreground-
background contrast in Assignments section.

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
  JIRA and compare with the updated UI to make
  sure it is fixed.
- The webAIM color contrast tester can be found
  here: http://webaim.org/resources/contrastchecker/

Change-Id: I0fd9441b5e23137be2ca17970f52a7194d378233
Reviewed-on: https://gerrit.instructure.com/41769
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>