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

install/lib/installer_base.lib.php

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

interface/lib/classes/simplepie.inc.php

interface/web/dns/dns_dmarc_edit.php

server/lib/app.inc.php

server/lib/classes/cron.d/100-monitor_rkhunter.inc.php

server/lib/classes/db_mysql.inc.php

server/plugins-available/apps_vhost_plugin.inc.php