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
if no price is set in shopping_cart or this new display_price field, isn;t the course "Free" as a matter of course? So displaying "Free" should be fine, no? (It's especially useful, as it sets a precedent on where to look for price of course on ALL Open edX course about pages...)