Unsubscribe URL on notification emails won't work for work item epics

Related Sentry error

SentNotificationsController#unsubscribe needs to be able to handle sent_notifications for group level issues. We need to fix path generation in https://gitlab.com/gitlab-org/gitlab/-/blob/d0967bc706e1f5791eef293b2e291914f60c261a/app/controllers/sent_notifications_controller.rb#L87