Fixed connection issue in mail account import tool.

MySQL connect with client flags. Fixes #4558

See merge request ispconfig/ispconfig3!798

Also update sys_user language if it was specified through API client_update

See merge request ispconfig/ispconfig3!797

re-creation for dkim keys fails (Fixes #5061)

See merge request !796

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.

Fix #select_server a that is affected by #sidebar a

See merge request ispconfig/ispconfig3!786

Use Acme v2 if supported

See merge request ispconfig/ispconfig3!787

Update letsencrypt.inc.php as per the discussion at ispconfig/ispconfig3#5030. Already pr'ed and merged in master via ispconfig/ispconfig3!778. Perhaps can be merged with stable too?

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 !766. Perhaps can be merged with stable too?

Add description text for logging options.

update logrotate

See merge request ispconfig/ispconfig3!784

Stable 3.1

See merge request ispconfig/ispconfig3!783