[Ticket/12701] Add events to user_add function

Add two new events to user_add function in ./includes/functions_user.php.

core.user_add_before - allow modification of submitted to function data.
Returns:
@var array $user_row - user_row array - User array
@var array $cp_data - cp_data array - CPF array

core.user_add_after - return user_id, user_row and cp_data after user registration
Returns:
@var int $user_id - user_id of the new user
@var array $user_row - user_row array - User array
@var array $cp_data - cp_data array - CPF array

Justification:
Allow extensions to parse data before and after creation of the user.

PHPBB3-12701