Color contrast on announcement summary text

fixes CNVS-13723
fixes CNVS-13724

Testing plan:

- Make sure you have an announcement
- Go to Announcements
- The summary text and date will now pass Webaim contrast ratios
for both normal UI and High Contrast UI
Normal UI Change: http://cl.ly/image/0h103W2G2a0U
High Contrast Change: http://cl.ly/image/3F3Y2U26313F

Change-Id: If51498ead1cb7cc2315f59d56bc1381425da5b7d
Reviewed-on: https://gerrit.instructure.com/41068
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>