Twenty Fifteen: add styles for multisite registration forms.

Props valendesigns, fixes #30776.
Built from https://develop.svn.wordpress.org/trunk@30995


git-svn-id: http://core.svn.wordpress.org/trunk@30977 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: pinking shears.

Built from https://develop.svn.wordpress.org/trunk@30996


git-svn-id: http://core.svn.wordpress.org/trunk@30978 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: fix missing style for menu in small screens.

Props valendesigns, fixes #30857.
Built from https://develop.svn.wordpress.org/trunk@30997


git-svn-id: http://core.svn.wordpress.org/trunk@30979 1a063a9b-81f0-0310-95a4-ce76da25c4cd