proposal Fix #6692 and #2962 add backup interval setting for databases

Currently there is only one setting for backup_interval for a site in ISPConfig UI.

However a seperate backup_interval for databases exist.

There is a use case where you want/need few backups of either web or database.

Also if you have a test database for example, you may wish to have no backup at all.

Either due to free space / quota constraints, barely changing data or other reasons.

Therefore the setting should be specific to a database.

The current situation is:

• when creating or updating a database, the backup interval from the website is used.
• when a domain is updated all databases are set to the same interval
• in some cases, maybe using api, the settings can become inconsistent https://forum.howtoforge.com/threads/backups.92260/#post-455937