RTL: Don't flip thumbnail positions in Insert Media window.
Restores pre-3.8 behavior.

props siobhyb for the initial patch.
fixes #26461.
Built from https://develop.svn.wordpress.org/trunk@26748


git-svn-id: http://core.svn.wordpress.org/trunk@26636 1a063a9b-81f0-0310-95a4-ce76da25c4cd