Handle deletions in Prismic media library backups

[gestchild]

We currently back up all the media assets held in Prismic on a daily basis. See infrastructure/prismic-snapshots/README.md

However, we don't account for deletions.

We get a list of all the assets as the first step in the backup process, so we could compare the previous list to the latest list and delete any assets that are no longer present. Or compare the list directly with the assets in S3