added 2 new endpoints
* /api/v1/summits/{id}/external-orders/{external_order_id}
to read external orders ( from eventbrite api)
* /api/v1/summits/{id}/external-orders/{external_order_id}/external-attendees/{external_attendee_id}/confirm
to confirm, and become an attendee
Change-Id: I1de5cf51fffdb45a9220190f412b3a121377b023