Add a backup site feature

It would need an API to backup a site (the command would only be accessible for site having Admin permissions), and it would do a complete copy of the site's directory and later if you want you could browse the site's backup. And if you make a new backup, it could do two things:
- override the old backup
- create another backup, in this case the backups would need to be timestamped, and it could also be incremental backups, it would decrease the size significantly

Me, I prefer the second solution