(WIP) Purchased course refund message when user unregister them

@cdodge Here is the [EX-23] changes. Kindly review them. let me know if anything else required in it. 
Thanks.
I think we need a little more logic here, because what we have is a paid course that is either in a refund period (yet to be defined) or not.

So something like (in pseudo-code)

%if is_paid_course and show_refund_option
    ... show that refund is available
%elif is_paid_course and not show_refund_option
    ... say that the unregistration is not refundable
:
:

Also - for now until we can allow course administrators to define a refund period - for paid for courses show_refund_option should be False. This feature is to get us in a position to start to support that.