let front-end properly react to conversion job status updates from revamped conversion service

Created by: das-g

• define what job status changes should cause what actions in the front end
  • Tracking job stati in Export model instances for later display (upon user request)
    • identify possible states and allowed transitions
  • Notification of orderer by on-screen messages
    • probably for all or most status changes of individual exports / jobs
  • Notification of orderer by email
    • probably if the overall extraction order enters specific stati (computed from its exports' stati)
      • Which ones? Only the final ones (finished or failed) as well as timed out?
      • How to compute the overall status?
• find out what status update notification the conversion service now sends to the callback URL
  • If some of those needed for the actions found above aren't sent by the service now, file another issue to make the service send them.