Add new entry point for extending/reactivating trial button

ee/app/assets/javascripts/pages/groups/billings/index.js

 import { shouldQrtlyReconciliationMount } from 'ee/billings/qrtly_reconciliation';
 import initSubscriptions from 'ee/billings/subscriptions';
+import { shouldExtendReactivateTrialButtonMount } from 'ee/trials/extend_reactivate_trial';
 import PersistentUserCallout from '~/persistent_user_callout';
 
 PersistentUserCallout.factory(document.querySelector('.js-gold-trial-callout'));
 initSubscriptions();
+shouldExtendReactivateTrialButtonMount();
 shouldQrtlyReconciliationMount();

ee/app/views/shared/billings/_billing_plan_header.html.haml

@@ -34,3 +34,7 @@
   - if show_start_free_trial_messages?(namespace)
     - glm_content = namespace_for_user ? 'user-billing' : 'group-billing'
     %p= link_to 'Start your free trial', new_trial_registration_path(glm_source: 'gitlab.com', glm_content: glm_content), class: 'btn btn-confirm gl-button', data: { qa_selector: 'start_your_free_trial' }
+
+  - if show_extend_reactivate_trial_button?(namespace)
+    .gl-mt-3
+      .js-extend-reactivate-trial-button.gl-mt-3{ data: extend_reactivate_trial_button_data(namespace) }