Merge branch 'stable-3.1' into 'stable-3.1'

interface/lib/app.inc.php

interface/lib/classes/db_mysql.inc.php

interface/lib/classes/session.inc.php

...rface/web/themes/default/assets/stylesheets/ispconfig.css

...face/web/themes/default/assets/stylesheets/ispconfig.sass

interface/web/tools/dns_import_tupa.php

interface/web/tools/import_vpopmail.php

interface/web/tools/templates/dns_import_tupa.htm

interface/web/tools/templates/import_vpopmail.htm

server/lib/app.inc.php

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

server/lib/classes/cron.d/200-ftplogfiles.inc.php

server/lib/classes/db_mysql.inc.php

server/server.php