this means that if the task is accidentally ran twice (eg we autoscale
notify-celery-worker-beat to 2), it won't send letters twice.
Additionally, update some function names and config variables to make
it clear that they are referring to letter jobs, rather than all letter
content
- brings boto S3 into new AWS folder
- CSV processing utils method
Rejigs the jobs rest endpoint - removes some now unused endpoints,
Calls to the task with the job, job processing in task, delegating SMS calls to the sms task
