Ensure that theme customizer buttons don't overlap on small screens; allow small-screen theme popup to use the full height of the viewport since the adminmenu is no longer fixed; ensure that buttons in the theme customizer wrap as efficiently as they can on mobile. Fixes #26361, props mattheu, iammattthomas.
Built from https://develop.svn.wordpress.org/trunk@26592


git-svn-id: http://core.svn.wordpress.org/trunk@26482 1a063a9b-81f0-0310-95a4-ce76da25c4cd