Move rendering of checkout button to a separate file and wrap it in a…

[vedanshujain]

… filter.

This will allow us to add customize the checkout form if needed.

[coreymckrill]

Couple of things:

• I'm not sure that "checkout form" is the best name, semantically, since it doesn't include all the fields you would normally fill out when doing a checkout. I'd probably go with something more specific, like "payment options".

• apply_filters is generally used to modify a value that is given as one of the parameters. In this case, we're not modifying $total, we're just using it to render HTML. So do_action would probably be more appropriate.

• That said, another way to approach this might be to store the output buffer of the rendered HTML in a variable, and then feed that into an apply_filters call before echoing it.