Preserve body structure in send_notification_email

Add new file 100-monitor_kernel_version.inc.php to stable-3.1 in relation to recently merged commits of ispconfig/ispconfig3!790 and ispconfig/ispconfig3!791.

There is a fix to the filename for master submitted via commit ispconfig/ispconfig3!804 that is still pending.

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?

Added a third parameter to $app->log() function in server part to turn off logging to database and use this option in mysql lib to not log any server side SQL errors to the database.

every cron execution. The logfile can be disabled by using -NoLog