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
@caesar2164 it's a UX/Product question, of do we really want this to be something that is said explicitly on every course about page. Please do me a favor and add some screenshots to this PR description with all the changed screens this adds (ie a free course, a course that costs, and any other changed UX).