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

install/lib/installer_base.lib.php

install/tpl/apache_apps.vhost.master

interface/lib/classes/sites_database_plugin.inc.php

interface/lib/plugins/sites_web_vhost_domain_plugin.inc.php

interface/web/sites/web_vhost_domain_edit.php

server/conf/apache_apps.vhost.master

server/conf/nginx_vhost.conf.master