Switch to one column on the edit/write screens at 850px (the editor becomes...

Switch to one column on the edit/write screens at 850px (the editor becomes unusable at narrower screens and two columns), see #20015

git-svn-id: https://core.svn.wordpress.org/trunk@20335 1a063a9b-81f0-0310-95a4-ce76da25c4cd