Add settings for deletion of backup data

[danmarsden]

It should be possible for a site to delete files stored in backups that have not been "seen" for a time period, eg 12 months after the file was last seen, delete it from the external backup storage.

[danmarsden]

This needs some further thought and we want to prevent deletion of backup data - the main user may not have access to delete objects, we don't want accidental deletion of backup data to be a possibility.