Align login box, messages, navigation links and header image on the login...

Align login box, messages, navigation links and header image on the login screen, props SergeyBiryukov and helenyhou, fixes #19331

git-svn-id: https://core.svn.wordpress.org/trunk@19414 1a063a9b-81f0-0310-95a4-ce76da25c4cd