[requires backend updates] Simplify status logic (after combining latest-job and status endpoints)

[tcompa]

Related to #755.

If we had a single backend endpoint with a response like:

{
  "job_status": "submitted",
  "statuses": {
    "65": "done",
    "66": "submitted"
  }
}

this would simplify the internal logic for which calls to make in background to update the workflow-task status.

Note: it'd be simpler to do this after we deprecate the status-legacy endpoint.