Adds paypal button whose values get set dynamically

~~So here's what I've found. The implementation below would work if we had a Merchant ID. We do not, so when we try to use it an error is shown. The other thing is to add a button but those seem super specific. Then we could create a custom form but that sounds like a bad User Experience. So all that being said I think we go with Stripe for now (it works in US/Europe/Australia). Once we get our first few themes (our asked to support international), we set up the bank account, etc.~~ cc @wallawe