Color contrast - Page history/Revision history

fixes CNVS-16037

This addresses color contrast issues for the revisions sidebar.

Test plan:

- Go to a page and in the settings dropdown for that page click,
View History
- The sidebar on the right is where our focus is and fixes
these areas (note: screenshots are just for reference):

Latest Revision text (selected)
http://screencast.com/t/KRQcScJwThb

Latest Revision text (not selected)
http://screencast.com/t/2buf1XI9

"X" to close (hover)
http://screencast.com/t/Q118UmV9LeBG

Same as Latest label
http://screencast.com/t/eglqqp7ZXL

Change-Id: I636311c65cb3b1589e46eb9e850d67f62f5e17c6
Reviewed-on: https://gerrit.instructure.com/45066
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>