Merge branch 'ftp_statistics' into 'stable-3.1'

Ftp statistics feature

Hi,
I add the ftp traffic support for pure-ftpd.
- Ftp statistics display per hostname in site section (traffic in and out)
- Ftp_traffic table (creation and update patch upd_0081.sql)
- Cronjob
 - Actually pure-ftpd log rotation are configured with /etc/logrotate.d/pure-ftpd-common
 - Maybe replace that per Ispconfig log rotation ?
- Api function for ftp traffic

Thanks

See merge request !266