Merge branch 'stable-3.1'

install/lib/installer_base.lib.php

install/lib/update.lib.php

install/sql/incremental/upd_0083.sql

install/sql/incremental/upd_dev_collection.sql

install/tpl/apache_ispconfig.vhost.master

interface/lib/classes/client_templates.inc.php

interface/lib/classes/remote.d/sites.inc.php

interface/lib/classes/tform_base.inc.php

interface/lib/classes/tools_sites.inc.php

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

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

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

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

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

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

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

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

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

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

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