Use a ModelList for lists of jobs

This follows the pattern of what we’ve done with services, users and
events.

It gives us a way of neatly instantiating a model for each item in the
list we get back from the API and reduces the complexity of the view
layer code.

Now is a good time to do this because we’re going to be making a bunch
of changes to the jobs pages, and those changes will be easier to code
and understand with a sensible model behind them.

app/statistics_utils.py

@@ -78,19 +78,3 @@ def statistics_by_state(statistics):
             'failed': statistics['emails_failed']
         }
     }
-
-
-def get_failure_rate_for_job(job):
-    if not job.get('notifications_delivered'):
-        return 1 if job.get('notifications_failed') else 0
-    return (
-        job.get('notifications_failed', 0) /
-        (job.get('notifications_failed', 0) + job.get('notifications_delivered', 0))
-    )
-
-
-def add_rate_to_job(job):
-    return dict(
-        failure_rate=(get_failure_rate_for_job(job)) * 100,
-        **job
-    )