OOjs UI: Display windows on top of the fixed header

Workaround copied from Vector.

Bug: T183284
Change-Id: I7a4847f8d7c81e30610964762d6e591d753fa207

resources/ooui.less

@@ -1,3 +1,8 @@
+.skin-timeless .oo-ui-windowManager-modal > .oo-ui-dialog,
+.skin-timeless .ve-ui-overlay-global {
+	z-index: 101;
+}
+
 .oo-ui-defaultOverlay {
 	z-index: 101;
 	font-size: 0.95em;