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

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

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

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

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

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

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

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

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

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

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

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

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