Don't allow setting of amounts client side

[thorsten-stripe]

It's generally not recommended to set the amount client side without validating the input on the server.

I think it might make sense to limit creation of Checkout sessions to usage with pre-created price IDs and disallow creation of PaymentIntents for now.

• Checkout sessions: only allow line_items[].price and disallow setting of line_items[].price_data
• PaymentIntents: don't yet support line_items therefore we should disallow creation of PaymentIntents client-side or we should add some functionality where you can feed a product.json file to NextStripe(req, res, options) via the options and then rather than passing in an amount client-side, you pass in your product id and the library then finds your product form the json and gets the amount from there.