Extract methods in admin menu component

Now that there's no naming confict with the helper method used in the
ProposalsDashboardHelper, we can easily simplify the view, moving the
logic to the Ruby class.

app/components/admin/menu_component.rb

@@ -57,4 +57,12 @@ class Admin::MenuComponent < ApplicationComponent
       controller_name == "local_census_records" ||
         (controller_name == "imports" && controller.class.parent == Admin::LocalCensusRecords)
     end
+
+    def messages_menu_active?
+      messages_sections.include?(controller_name)
+    end
+
+    def messages_sections
+      %w[newsletters emails_download admin_notifications system_emails]
+    end
 end