(WIP) Modify e-commerce instructor tab to show a total amount above the coupon...

@cdodge kindly review the change set for [Ex-29]. Let me know if anything else required in it. 
Thanks.
Actually i was confused with the same implementation in class method "total_amount" of orderitem model. where we were calculating the total amount in cart. This approach is not good and i agree.
I will update the code tomorrow and will calculate the sum using aggregation function in query. Thanks for highlighting that.