Add price field to studio & logic to show/hide it

Add cosmetic price field to studio & logic to show it

- Price field added to Advanced Settings
- Function that checks for price to display like this:
    - If display_price is set, set price var to that
    - If shopping_cart price is set, overwrite price var
    - if price var is set, return that, otherwise return "Free"
- Test added to check that the function outputs correct price
I don't even know if edx.org wants to have "Price: Free" added to all of their courses.  The precedent it sets is that it raises the question in students' minds about which courses might not be free.  Do we have someone in the product organization who can weigh in on this?  @griffresch?