Force LTR direction for code and plugin/theme editors.

props SergeyBiryukov, nivijah.
fixes #26666 for trunk.
Built from https://develop.svn.wordpress.org/trunk@26888


git-svn-id: http://core.svn.wordpress.org/trunk@26771 1a063a9b-81f0-0310-95a4-ce76da25c4cd