Customize '3 week app update' special backup retention policy for nextcloud

[ethanxrosen]

Use case-
Nextcloud updates very frequently, meaning that even if you only intend to keep 1-2 app backups at a time, you can end up with 5 or 6 due to cloudron's 3 week app update policy: https://docs.cloudron.io/backups/#retention-policy

If you have a 1tb nextcloud, this triples the storage costs / box space and adds up really quick. I think this is especially unnecessary when many people will also have a physical drive synced via the nextcloud client as a third backup.

You can delete these backups manually from your storage provider if you remember to, but it would be awesome to have an option to customize this, so people can enable automatic updates+backups of large apps in cloudron without driving too much cost.