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

install/lib/installer_base.lib.php

interface/lib/classes/db_mysql.inc.php

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

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

interface/web/admin/system_config_edit.php

interface/web/client/templates/client_edit_limits.htm

interface/web/dashboard/dashboard.php

interface/web/sites/ajax_get_ip.php

interface/web/themes/default/assets/javascripts/ispconfig.js

server/conf/php_fpm_pool.conf.master

server/cron.php

server/lib/classes/cronjob.inc.php

server/plugins-available/apache2_plugin.inc.php

server/plugins-available/firewall_plugin.inc.php

server/plugins-available/nginx_plugin.inc.php