Administration: Fix link colors for admin schemes that personalize it using...

Administration: Fix link colors for admin schemes that personalize it using the $link SASS variable.

The new "moderne" admin scheme color personalizes the link color using the $link SASS variable.
The link color doesn't apply consistently in the admin to all links, especiall button links.

Fixes #50536.

Built from https://develop.svn.wordpress.org/trunk@48332


git-svn-id: https://core.svn.wordpress.org/trunk@48101 1a063a9b-81f0-0310-95a4-ce76da25c4cd