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

install/sql/incremental/upd_dev_collection.sql

install/sql/ispconfig3.sql

interface/web/admin/form/server_config.tform.php

interface/web/admin/lib/lang/ar_server_config.lng

interface/web/admin/lib/lang/bg_server_config.lng

interface/web/admin/lib/lang/br_server_config.lng

interface/web/admin/lib/lang/ca_server_config.lng

interface/web/admin/lib/lang/cz_server_config.lng

interface/web/admin/lib/lang/de_server_config.lng

interface/web/admin/lib/lang/dk_server_config.lng

interface/web/admin/lib/lang/el_server_config.lng

interface/web/admin/lib/lang/en_server_config.lng

interface/web/admin/lib/lang/es_server_config.lng

interface/web/admin/lib/lang/fi_server_config.lng

interface/web/admin/lib/lang/fr_server_config.lng

interface/web/admin/lib/lang/hr_server_config.lng

interface/web/admin/lib/lang/hu_server_config.lng

interface/web/admin/lib/lang/id_server_config.lng

interface/web/admin/lib/lang/it_server_config.lng

interface/web/admin/lib/lang/ja_server_config.lng