Merge branch 'stable-3.1'

install/dist/conf/debiantesting.conf.php

install/dist/lib/ubuntu1604.lib.php

install/lib/install.lib.php

install/lib/installer_base.lib.php

install/lib/update.lib.php

interface/lib/classes/tform_base.inc.php

interface/web/admin/templates/system_config_dns_edit.htm

interface/web/dashboard/lib/lang/pl_dashlet_modules.lng

interface/web/dns/dns_import.php

interface/web/sites/database_quota_stats.php

interface/web/sites/templates/database_user_edit.htm

interface/web/sites/web_vhost_domain_edit.php

server/lib/classes/cron.d/900-letsencrypt.inc.php

server/lib/classes/monitor_tools.inc.php

server/lib/classes/system.inc.php

server/plugins-available/apache2_plugin.inc.php

server/plugins-available/cron_plugin.inc.php

server/plugins-available/mail_plugin.inc.php

server/plugins-available/mail_plugin_dkim.inc.php

server/plugins-available/nginx_plugin.inc.php