UX: the ability to hide the admin header (#30175)

Some pages like new/edit item should not display admin header. New attribute called `@shouldDisplay` was added.

As a proof of concept, the flags page was updated.

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

@@ -6,6 +6,14 @@ module PageObjects
       def has_tabs?(names)
         expect(page.all(".admin-nav-submenu__tabs a").map(&:text)).to eq(names)
       end
+
+      def visible?
+        has_css?(".admin-page-header")
+      end
+
+      def hidden?
+        has_no_css?(".admin-page-header")
+      end
     end
   end
 end