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 
I think you need an `andCallThrough()` here, otherwise the test will trivially pass (it was enabled before and is also enabled after).

It would also be okay to *not* stub this out and verify that the button is enabled after the failure.  Since you have another test that verifies that it's disabled on the "success" path, the two tests are jointly sufficient.