Backup service: add option to delete backup from local server

Backups can take a lot of space. Once they are copied to the remote server, there is usually no need to keep them in the local server. This could be done with a new setting on the BackupService.

:warning: If the copy fails (for example if there is not enough space on the remote server), we should not delete the backup on the local server.

Start by locating the BackupService entry point and tracing how backups are copied to the remote server. Define where the new local-deletion setting belongs and verify that a failed copy leaves the local backup intact; done means successful copies can remove local files while failed copies do not.

Written by the indexing model from the issue text.