Per-app backup scheduling and more options for more frequent backups

[atridad]

1. All apps can have their own backup schedule and retention policy that overrides the server backup settings
2. These backups can have intervals down to 30 minutes (for stuff like Minecraft servers)
3. App backups can also be triggered manually
4. Apps with their own schedule are not included in the system backup

Is something like this feasable? My example above is for minecraft servers but like it can be useful for client sites and more critical services that may need more strict retention policies or backup frequency.

[jdaviescoates]

@atrilahiji also, just having per-app setting would enable to do eg. Nextcloud using rsync and other apps using tar which I'd really like to be able to do (and has been previously requested and was even included in a "what's coming in Cloudron X" post but then never actually made it in).