registration code checkout flow

Redirecting to dashboard after free user enrollment.
added course price set/view/edit functionality

added CoursmodeArchive model to save the old prices history
Merge pull request #4526 from edx/asadiqbal08/coupon_plus_registration_code_checkout

Registration code checkout flow
Merge pull request #4535 from edx/muhhshoaib/Ex-41-As-a-FinanceAdmin-user,-I-would-like-to-set/view/edit-the-list-price-for-my-course

added course price set/view/edit functionality
refund message for purchased courses

Add little more logic for paid courses