Fixed delete WiSP user to delete group associations

More error checking

Failing to insert user, needs fixing

* Changed attribute name SMRadius-Capping-Time-Limit to SMRadius-Capping-UpTime-Limit, this makes way for another feature to be added soon

Insert into wisp_userdata location

Fixed $_POST['user_id'] defined in user_attributes.php

Improved security
Improved readability