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
There is no logic here to decide to display a price.  This always displays a price, but it can be "Free".  Do we want to add "Price" to every course about page on every Open edX site?