[ticket/15492] Fix permission role combobox in RTL

Varius fixes to css and js for correct
displaying permission role combbox in RTL mode

PHPBB3-15492

phpBB/assets/javascript/core.js

@@ -1331,6 +1331,8 @@ phpbb.toggleDropdown = function() {
 			$this.css({
 				marginLeft: 0,
 				left: 0,
+				marginRight: 0,
+				right: 0,
 				maxWidth: (windowWidth - 4) + 'px'
 			});