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
That seems like a rather significant change from how things currently are, yes? Do we have a sense as to whether or not community members would want this change appearing on their sites?