Added interface part for website backup function.

install/sql/ispconfig3.sql

install/tpl/server.ini.master

interface/web/admin/form/server_config.tform.php

interface/web/admin/lib/lang/en_server_config.lng

interface/web/admin/templates/server_config_server_edit.htm

interface/web/sites/form/web_domain.tform.php

interface/web/sites/lib/lang/en_web_domain.lng

interface/web/sites/templates/web_domain_backup.htm