UX: flag settings tab to follow UI guidelines (#28479)

Add settings tab to flags moderation page.

spec/system/page_objects/components/admin-header.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Components
+    class AdminHeader < PageObjects::Pages::Base
+      def has_tabs?(names)
+        expect(page.all(".admin-nav-submenu__tabs a").map(&:text)).to eq(names)
+      end
+    end
+  end
+end