Add A/B testing support to onboarding

Check if we're gitlab.com and the
:user_onboarding feature flag is enabled.

app/controllers/dashboard/projects_controller.rb

@@ -3,6 +3,7 @@
 class Dashboard::ProjectsController < Dashboard::ApplicationController
   include ParamsBackwardCompatibility
   include RendersMemberAccess
+  include OnboardingExperimentHelper
 
   prepend_before_action(only: [:index]) { authenticate_sessionless_user!(:rss) }
   before_action :set_non_archived_param