CRM-14856 - CRM_Profile_Form - Load postURL and add_to_group from DB (instead of POST)

Also:
 * Keep & use a single copy of the UFGroup in $this->_ufGroup.
 * Retain "gid" as a hidden field (for use in POST-backs).
CRM-14856 - CRM_Profile_Form_Edit - Reimplement cancelURL