Merge branch 'stable-3.1' into 'add-some-translations'

CONTRIBUTING.md

install/dist/tpl/gentoo/jk_init.ini.master

install/lib/installer_base.lib.php

install/sql/README.txt

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

install/tpl/jk_init.ini.master

install/tpl/security_settings.ini.master

install/tpl/server.ini.master

install/tpl/system.ini.master

interface/lib/classes/aps_guicontroller.inc.php

interface/lib/classes/custom_datasource.inc.php

interface/lib/classes/ispcmail.inc.php

interface/lib/classes/listform_actions.inc.php

interface/lib/classes/plugin_backuplist.inc.php

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

interface/lib/classes/sites_database_plugin.inc.php

interface/lib/classes/system.inc.php

interface/lib/lang/fi.lng

interface/lib/plugins/sites_web_vhost_domain_plugin.inc.php