Add basic responsive styles for button-small and button-large, override responsive button-large styles when used in the .interim-login iframe. Fixes #26363.
Built from https://develop.svn.wordpress.org/trunk@26533


git-svn-id: http://core.svn.wordpress.org/trunk@26426 1a063a9b-81f0-0310-95a4-ce76da25c4cd