Add scheduled task to find precompiled letters in wrong state

Added a task which runs twice a day on weekdays and checks for letters that have
been in the state of `pending-virus-check` for over 90 minutes. This is
just logging an exception for now, not trying to fix things, since we
will need to manually check where the issue was.

app/config.py

@@ -276,6 +276,11 @@ class Config(object):
             'schedule': crontab(hour=23, minute=00),
             'options': {'queue': QueueNames.PERIODIC}
         },
+        'check-precompiled-letter-state': {
+            'task': 'check-precompiled-letter-state',
+            'schedule': crontab(day_of_week='mon-fri', hour='9,15', minute=0),
+            'options': {'queue', QueueNames.PERIODIC}
+        },
     }
     CELERY_QUEUES = []