Merge branch 'develop' of https://git.ispconfig.org/ispconfig/ispconfig3 into develop

install/install.php

install/lib/installer_base.lib.php

install/lib/mysql.lib.php

install/sql/incremental/upd_0089.sql

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

install/tpl/mysql-virtual_email2email.cf.master

install/tpl/mysql-virtual_mailboxes.cf.master

install/uninstall.php

install/update.php

interface/lib/classes/db_mysql.inc.php

interface/lib/classes/tform_base.inc.php

interface/web/mail/form/mail_user.tform.php

interface/web/mail/lib/lang/ar_mail_user.lng

interface/web/mail/lib/lang/bg_mail_user.lng

interface/web/mail/lib/lang/br_mail_user.lng

interface/web/mail/lib/lang/ca_mail_user.lng

interface/web/mail/lib/lang/cz_mail_user.lng

interface/web/mail/lib/lang/de_mail_user.lng

interface/web/mail/lib/lang/dk_mail_user.lng