Merge branch 'stable-3.1' of git.ispconfig.org:ispconfig/ispconfig3 into stable-3.1

install/lib/installer_base.lib.php

install/lib/update.lib.php

install/sql/incremental/upd_0084.sql

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

install/tpl/nginx_ispconfig.vhost.master

interface/lib/lang/cz.lng

interface/lib/lang/dk.lng

interface/lib/lang/tr.lng

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

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

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

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

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

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

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

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

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

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

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