Only show the Express Checkout button if the rule's conditions evaluate to TRUE

Right now, the Express Checkout method may not be visible in the payment pane but still would show on the cart form, because we only check to see if it's enabled before altering the form. Part of the problem is the function commerce_paypal_ec_enabled() doesn't return anything that would help us make such a check. We should update that function per its @todo notes and change it to actually return the first enabling rule whose conditions evaluate to TRUE for the given order. This would solve our form issue tangentially and provide us with a meaningful return value for other potential use cases of the function.