Muhhshoaib/ex 72 wl purchasers provide optional information

@chrisndodge We have added the functionality related to Ex-72. In this PR we have added a new billing page to capture the billing information for order_type = 'business'. We have a added a new column in the Order Model to differentiate between business and personal orders. 

To cater for the requirement of financial download (ex-51) we have added a new CSV file in the sales tab that includes only the Sales Information related to the Order purchases.

In this PR we have also generated the registration codes against the order when order_type = 'business'.

To test this PR 
uncomment these lines 360 and 361 
https://github.com/edx/edx-platform/blob/d5200f75dfbb7321fcca1a4f0794288f188750fb/lms/djangoapps/shoppingcart/views.py#L360

The URL for the billing details page is /shoppingcart/billing_details

Kindly review the changes
Thanks