Use Codex styles for buttons in language overlay

* Replace all instances of mw-ui markup with Codex markup
* Use @wikimedia/codex as it is available in 1.39
and codex-search-styles is not.

Bug: T340255
Change-Id: Id85cf3b4af782c0d93bfa3bed5f3e6cbed856f1b

resources/css/ext.uls.displaysettings.less

@@ -9,7 +9,8 @@
 .uls-button-group {
 	display: inline-block;
 	position: relative;
-	padding: 0;
+	// The 1px are needed to handle overflow in .cdx-button-group in 1.39
+	padding: 1px 0 0 1px;
 	left: -20px;
 	z-index: 1;
 }