Implements #5290 in stable branch: Nginx - Allow certbot acme challenge on auth_basic protected websites

Fix: dont use strict comparison to install server plugin

See merge request ispconfig/ispconfig3!891

Fixed: WARNING - assumed 'LOGLEVEL_WARNING' (this will throw an Error in a future version of PHP) in /usr/local/ispconfig/server/lib/classes/services.inc.php on line 44

Added INSTALLER_UPDATE constant in update.php to allow easier detection if installer run is an update or new installation.

stats Folder created even when Statistics set to none - Fixes #4977

See merge request ispconfig/ispconfig3!885

Config option for jailkit authorized keys

See merge request ispconfig/ispconfig3!867

Sync copies of db_mysql.inc.php

See merge request !883

Feature/password strength checker for aps

See merge request !881

Fixes #4972 Domain in spamfilter_user settings not punicode encoded

Use 127.0.0.1 as APS db host if php_fpm_chroot, fixes #5235

See merge request ispconfig/ispconfig3!879

Don't hardcode /web folder for php-fpm-chroot, fixes #3220

See merge request ispconfig/ispconfig3!878

Stable 3.1

See merge request !877