Add a cronjob to clean our GCS object storage #1004
Description
When an attachment is updated on an existing record, we don't delete the old file.
First, because attachments are added on the workspace bucket, which is not yet public.
And also we consider that a client may still have the old record in its local cache, and thus could still want to download the old file.
Nevertheless, kinto-attachment has been in production for almost 10 years and we NEVER purged old attachments. There are attachments in there that were attached to records that were deleted 8 years ago.
We have to find a threshold (1 year?) and delete all old attachments that are not referenced by any record.