Move stats to the model

The API clients should just deal with calling the API and returning the
data from it.

Inferring things from the data is more logically done at the model
layer. But we couldn’t do that before, because we didn’t have a model
layer for jobs.

tests/app/test_statistics_utils.py

@@ -128,25 +128,25 @@ def test_service_statistics_by_state():
     (1, 4, 20)
 ])
 def test_add_rate_to_job_calculates_rate(failed, delivered, expected_failure_rate):
-    resp = Job(
-        {
-            'notifications_failed': failed,
-            'notifications_delivered': delivered,
-            'id': 'foo'
-        }
-    )
+    resp = Job({
+        'statistics': [
+            {'status': 'failed', 'count': failed},
+            {'status': 'delivered', 'count': delivered},
+        ],
+        'id': 'foo',
+    })
 
     assert resp.failure_rate == expected_failure_rate
 
 
 def test_add_rate_to_job_preserves_initial_fields():
-    resp = Job(
-        {
-            'notifications_failed': 0,
-            'notifications_delivered': 0,
-            'id': 'foo'
-        }
-    )
+    resp = Job({
+        'statistics': [
+            {'status': 'failed', 'count': 0},
+            {'status': 'delivered', 'count': 0},
+        ],
+        'id': 'foo',
+    })
 
     assert resp.notifications_failed == resp.notifications_delivered == resp.failure_rate == 0
     assert resp.id == 'foo'