Add an endpoint to cancel a job

If you schedule a job you might change your mind or circumstances might
change. So you need to be able to cancel it. This commit adds a `POST`
endpoint for individual jobs which sets their status to `cancelled`.

This also means adding a new status of `cancelled`, so there’s a
migration…

app/models.py

@@ -308,6 +308,7 @@ JOB_STATUS_IN_PROGRESS = 'in progress'
 JOB_STATUS_FINISHED = 'finished'
 JOB_STATUS_SENDING_LIMITS_EXCEEDED = 'sending limits exceeded'
 JOB_STATUS_SCHEDULED = 'scheduled'
+JOB_STATUS_CANCELLED = 'cancelled'
 
 
 class JobStatus(db.Model):