Updates to the delete CSV file job to reduce the number of eligible jobs in any run

- previously this was unbounded, so it got all jobs older then 7 days. In excess of 75,000 🔥
- this meant that the job took (a) a long time and (b) a lot memory and (c) doing the same thing every day

These changes mean that the job has a 2 day eligible window for jobs, minimising the number of eligible jobs in a run, whilst still retaining some leeway in event if it failing one night.

In principle the job runs early morning on a given day. The previous 7 days are left along, and then the previous 2 days worth of files are deleted:

so:
runs on
31st
30,29,28,27,26,25,24 are ignored
23,22 jobs here have files deleted
21 and earlier are ignored.

app/celery/scheduled_tasks.py

@@ -10,7 +10,7 @@ from app.aws import s3
 from app import notify_celery
 from app import performance_platform_client
 from app.dao.invited_user_dao import delete_invitations_created_more_than_two_days_ago
-from app.dao.jobs_dao import dao_set_scheduled_jobs_to_pending, dao_get_jobs_older_than
+from app.dao.jobs_dao import dao_set_scheduled_jobs_to_pending, dao_get_jobs_older_than_limited_by
 from app.dao.notifications_dao import (
     delete_notifications_created_more_than_a_week_ago,
     dao_timeout_notifications,
@@ -28,7 +28,7 @@ from app.celery.tasks import process_job
 @notify_celery.task(name="remove_csv_files")
 @statsd(namespace="tasks")
 def remove_csv_files():
-    jobs = dao_get_jobs_older_than(7)
+    jobs = dao_get_jobs_older_than_limited_by()
     for job in jobs:
         s3.remove_job_from_s3(job.service_id, job.id)
         current_app.logger.info("Job ID {} has been removed from s3.".format(job.id))