Don’t show cancelled jobs anywhere

The information about a job doesn’t make sense if a job is cancelled.

We could change the information to reflect that the job won’t be sent/
wasn’t sent/was cancelled, but there’s nothing you can really do with
this info.

So instead let’s:
- hide cancelled jobs from the dashboard
- hide cancelled jobs from the jobs page
- 404 the page if the user tries to click back enough times to hit the
  job page

app/main/views/dashboard.py

@@ -120,7 +120,7 @@ def get_dashboard_partials(service_id):
         job for job in jobs if job['job_status'] == 'scheduled'
     ], key=lambda job: job['scheduled_for'])
     immediate_jobs = [
-        job for job in jobs if job['job_status'] != 'scheduled'
+        job for job in jobs if job['job_status'] not in ['scheduled', 'cancelled']
     ]
     service = service_api_client.get_detailed_service(service_id)