Make login styles standalone. see #12506.


git-svn-id: https://develop.svn.wordpress.org/trunk@27199 602fd350-edb4-49c9-b593-d223f7449a82
Login styles do not depend on common.css. see #12506.


git-svn-id: https://develop.svn.wordpress.org/trunk@27200 602fd350-edb4-49c9-b593-d223f7449a82