Account for missing scheduled for field

Jobs have a `scheduled_for` field. Single letter uploads don’t.

At the moment we treat both of them as `Job`s. So the `Job` model needs
to account for when the `scheduled_for` field is missing.

app/models/job.py

@@ -26,7 +26,6 @@ class Job(JSONModel):
         'notification_count',
         'job_status',
         'created_by',
-        'scheduled_for',
     }
 
     @classmethod
@@ -45,6 +44,10 @@ class Job(JSONModel):
     def scheduled(self):
         return self.status == 'scheduled'
 
+    @property
+    def scheduled_for(self):
+        return self._dict.get('scheduled_for')
+
     def _aggregate_statistics(self, *statuses):
         return sum(
             outcome['count'] for outcome in self._dict['statistics']