Increase text contrast in Courses dropdown and Discussions
Fixes: CNVS-14304

Test plan:
- Check the main Courses dropdown in regular-contrast mode
  and verify that the subtitle text (Enrolled as: teacher)
  is now darker, like the screenshot I have posted in the
  JIRA
- Create a discussion and a couple of replies
- Verify that the Reply link is now darker, like on the
  screenshot I have attached to the JIRA. I used the
  ensure-color-contrast function, so the colors should
  now adhere to  WebAIM standards.

Change-Id: Id5deac49b5650370469e320e631b0c747dd1adb4
Reviewed-on: https://gerrit.instructure.com/39114
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>