RTL: Fix modal alignments.

* Prevent horizontal scrollbars in Chrome.
* URL fields should be always LTR.
* Force the plugin details to be LTR for all screens, see #19778.

fixes #29352.
Built from https://develop.svn.wordpress.org/trunk@29623


git-svn-id: http://core.svn.wordpress.org/trunk@29397 1a063a9b-81f0-0310-95a4-ce76da25c4cd