Add outlines and borders to mark the boundaries between the admin navigation menu and content and around adminbar submenus that are visible when Windows High Contrast Mode is enabled. This clarifies the page structure and makes high contrast mode easier to use.
Props wildworks, hbhalodia, sabernhardt, joedolson, rcreators.
Fixes#61616.
Built from https://develop.svn.wordpress.org/trunk@59041
git-svn-id: http://core.svn.wordpress.org/trunk@58437 1a063a9b-81f0-0310-95a4-ce76da25c4cd
