Make login styles standalone. see #12506.

Built from https://develop.svn.wordpress.org/trunk@27199


git-svn-id: http://core.svn.wordpress.org/trunk@27056 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Login styles do not depend on common.css. see #12506.

Built from https://develop.svn.wordpress.org/trunk@27200


git-svn-id: http://core.svn.wordpress.org/trunk@27057 1a063a9b-81f0-0310-95a4-ce76da25c4cd