Work Items list [BE] - Add Subscribe to Calendar/RSS Feed [Group]

• For calendar, makes a request to a URL like http://127.0.0.1:3000/groups/gitlab-org/-/issues.ics?due_date=next_month_and_previous_two_weeks&feed_token=glft-16e82da962187048b0e80ad416d38625a6286b5da882841accc9051cb3f34018-1&sort=closest_future_date