Add backup policy with DrupalProjectConfig

Part of https://gitlab.cern.ch/webservices/webframeworks-planning/-/issues/725

• Add a new CRD, DrupalProjectConfig, which defines which site is the "primary" site of the project
• Deprecate DrupalSite.spec.configuration.ScheduledBackups
• Declare primary site automatically when only 1 site is present
• Add DrupalSite.status.isPrimary, which reports if this site is the primary site
• Enable scheduled backups only for the primary site (but still allow the ScheduledBackups: "enabled" setting to override this and always allow backups)