Merge Adyen and Barclaycard Smartpay gateways

[bpollack]

These are actually the same gateway, and could theoretically be merged, but they currently implement nonoverlapping parts of the API, with different parameter names and behaviors. We should probably start by redefining Barclaycard Smartpay in terms of Adyen, moving any Barclaycard-specific functionality into Adyen, and then work with downstream users to deprecate the Barclaycard Smartpay gateway entirely.