Fixed connection issue in mail account import tool.

Add word "Kernel" to all respective language files in stable3.1.

$wb['monitor_serverstate_kernel_txt'] = 'Kernel';

In relation with the earlier merged commits of !790 
 and !791.

Added remote functions config_value_get, config_value_add, config_value_update, config_value_replace and config_value_delete to access the sys_config key / value store in ISPConfig from remote api.

Update ispconfig.css to fix #select_server a that is affected by #sidebar a padding of 10px. Already pr'ed and merged in master via ispconfig/ispconfig3!766. Perhaps can be merged with stable too?

Add description text for logging options.

- Fixed #5027 add support for PHP 7.1 and 7.2 on nginx servers in apps vhost.
- suppress some warnings in simplepie.inc.php

global option to enable pagespeed with nginx - allows to enable pagespeed even if no webserver-config is selected

Reseller is able to see a list of all clients and not just of his own clients in select field in the head of domain list when domain module is active.