Compress and bump wp-admin.css. CSS in [18867] and [18874] will need to be...

Compress and bump wp-admin.css. CSS in [18867] and [18874] will need to be explained and re-committed. see #18785.

git-svn-id: https://core.svn.wordpress.org/trunk@18912 1a063a9b-81f0-0310-95a4-ce76da25c4cd