Admin Menu: Auto fold the menu at 960px.

This gives us some more room at the post edit screen.

props vinod dalvi for initial patch.
fixes #28592.
Built from https://develop.svn.wordpress.org/trunk@29518


git-svn-id: http://core.svn.wordpress.org/trunk@29294 1a063a9b-81f0-0310-95a4-ce76da25c4cd