Use time to determine why notifications don’t exist

Notifications won’t exist for a job if:
- it’s just started
- it started a long time ago (older than the retention period)

We have a bug where:
1. Job starts processing, puts notifications on queue
2. Job finishes processing, sets status to `finished`
3. First notification gets picked up off the queue and put in the
   database

In between 2. and 3. it’s possible for a job to be finished, but also to
have no notifications. We’re saying this is because the notifications
have been deleted, whereas really it’s because they haven’t been created
yet.

This commit fixes that bug by introducing the concept of recency for
jobs.

‘Recent’ is defined as 1 day, which is:
- a lot longer than it takes to create any notifications
- a bit shorter than anyone’s retention time

N.B. `processing_started` is defined here:
879ba1d5f0/app/models.py (L1194)

It can be `None` for scheduled jobs that haven’t started yet.

app/models/job.py

@@ -48,6 +48,12 @@ class Job(JSONModel):
     def scheduled_for(self):
         return self._dict.get('scheduled_for')
 
+    @property
+    def processing_started(self):
+        if not self._dict.get('processing_started'):
+            return None
+        return datetime.strptime(self._dict['processing_started'][:-6], '%Y-%m-%dT%H:%M:%S')
+
     def _aggregate_statistics(self, *statuses):
         return sum(
             outcome['count'] for outcome in self._dict['statistics']
@@ -95,6 +101,17 @@ class Job(JSONModel):
     def finished_processing(self):
         return self.notification_count == self.notifications_sent
 
+    @property
+    def recently_created(self):
+        if not self.processing_started:
+            # Assume that if processing hasn’t started yet then the job
+            # must have been created recently enough to not have any
+            # notifications yet
+            return True
+        return (
+            datetime.utcnow() - self.processing_started
+        ).days < 1
+
     @property
     def template_id(self):
         return self._dict['template']