Increase color contrast of inbox unread count badge

Fixes: CNVS-14596

Note: the CSS that determines the background color
of the unread message badge also controls the blue
badges for dashboard activity, so this change will
also affect those elements.

Test plan:
- Send a message to make the unread messages badge
  appear in the user nav in the header
- Use the web inspector to get the HEX values for
  the badges in both default and high-contrast mode,
  and confirm they meet the WebAIM contrast standards
  of 3:1 in default UI and 4.5:1 in high contrast UI.

Change-Id: Idf0002c3fcf6e5869583d63cded02b62cb871b93
Reviewed-on: https://gerrit.instructure.com/39145
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>