Fix incorrect processing started time on jobs

We were doing the UTC to BST conversion twice, meaning the job was shown
as being started 2 hours later than UTC, and 1 hour later than actual
BST.

app/models/job.py

@@ -67,7 +67,7 @@ class Job(JSONModel):
     def processing_started(self):
         if not self._dict.get('processing_started'):
             return None
-        return utc_string_to_aware_gmt_datetime(self._dict['processing_started'])
+        return self._dict['processing_started']
 
     def _aggregate_statistics(self, *statuses):
         return sum(
@@ -124,7 +124,9 @@ class Job(JSONModel):
             # notifications yet
             return True
         return (
-            datetime.utcnow().astimezone(local_timezone) - self.processing_started
+            datetime.utcnow().astimezone(local_timezone) - utc_string_to_aware_gmt_datetime(
+                self.processing_started
+            )
         ).days < 1
 
     @property

tests/app/main/views/test_jobs.py

@@ -706,7 +706,7 @@ def test_should_show_updates_for_scheduled_job_as_json(
     assert 'Status' in content['notifications']
     assert 'Delivered' in content['notifications']
     assert '12:01am' in content['notifications']
-    assert 'Sent by Test User on 1 June at 5:00pm' in content['status']
+    assert 'Sent by Test User on 1 June at 4:00pm' in content['status']
 
 
 @pytest.mark.parametrize(