add api get user-acl

[l-with]

add api /get/user-acl

A possibility to get user-acl is necessary to integrate a terraform resource user-acl in the terraform-provider-mailcow.

The implementation is done by adapting the php code for get fail2ban in json_api.php and interpreting the function acl in functions.acl.inc.php

I hope, I produced functional code by this unconventional way adding functionality to the api. Please give me advice if this is not the case.

I offer to add the API specification in another pull request, therefore I need to try the new endpoint to have a look at the json output.