disable "go to payment button" on click (ECOM-643)

We currently have 2 ways of going to the payment page in cybersource, each with its own javascript implementation. I'll put in a ticket to combine them into one implementation--something we'll have to worry about is this `analytics` object, which only seems to be called during one of the payment methods. But I think that's probably out of scope of this PR, which just keeps people from double clicking on the "go to payment"  button.

@dianakhuang 

@wedaly 
If the AJAX call to create the order fails, the submit button should be re-enabled.  Otherwise, the user will see the error message but have no way to re-retry the request.