Color contrast - Individual announcement page

fixes CNVS-15673
fixes CNVS-15672

Since these two areas dealt with similar contrast issues, they're
tackled in one commit.

This adds the correct color ratios to meet accessibility.

Test Plan:

CNVS-15673

- Go to a specific discussion

- Click on "reply", make sure the ratio for the link "HTML Editor"
meets webaim standards. Note: we have taken away the blue background
purposely.
- Get the colors on the pubish timestamps; both on the main message
and on the individuals (see screenshots in jira story). These now
meet webaim ratio requirements.

CNVS-15672

- Go to announcements
- There should no longer be a gray background zebra striping on
some of the announcments, and this fixes the majority of ratio
contrast issues for HC mode (see jira ticket for screenshots)

Change-Id: Id76f1a6a34641af67237a0ec6298a4224786e922
Reviewed-on: https://gerrit.instructure.com/43814
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Vincent Gonzalez <vgonzalez@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>