Added tracking sub-statuses to sent shipment units

Shipment units now carry a more detailed sub_status while their status is sent, instead of staying opaque until they're delivered. Six new values reflect live carrier tracking updates: pick_up, in_transit, out_for_delivery, at_pickup_point, delayed and failed_delivery. See Tracking data for what each one means.

This only applies to a shipment unit's own sub_status - the shipment-level status/sub_status documented in Shipments overview are unaffected.