Implement unassigned agent status

[greglook]

When a job using an elastic agent profile has not been assigned to an agent yet, the "agent status link" will point to the phony agent-id unassigned with a query parameter for the job identifier. Since we track which agents were launched for which jobs, we should be able to surface information here about still-launching agents. This would give users a way to check for launch-time errors if their job goes unassigned for long periods of time.