Fixed #4425 Variable is initialized as string but later written to as array, throws fatal error in PHP 7.1

Fixed #4444 Error in the template htm /usr/local/ispconfig/interface/web/client/templates/reseller_edit_limits.htm

Fixed #4443 Error in the template htm /usr/local/ispconfig/interface/web/client/templates/client_edit_limits.htm

Fixed #4440 Error in the template htm /usr/local/ispconfig/interface/web/client/templates/client_template_edit_limits.htm

Improved the code that generates the ssh keys for clients and shell users and reorganized it into a central function.

Disable SSL cert check in ISPConfig mail user import tool to allow connections to ISPConfig servers with self-signed SSL certificates.