Feature/initial

This is the first iteration of the Adyen payment backend for Oscar.
It still needs documentation, which I am going to improve during the day, but I figure it's worth starting the review ASAP.

As for the other backends, it follows the <code>scaffold —> facade —> gateway</code> pattern.

Some of it might seem overengineered right now but this is to prepare a future, fuller implementation.



The previous line ensures that <code>feedback_code</code> will default to <code>Constants.PAYMENT_RESULT_ERROR</code>, so <code>feedback_message</code> should also, in turn, default to <code>self.FEEDBACK_MESSAGES[Constants.PAYMENT_RESULT_ERROR]</code>.
Useless comma removed.
:heavy_check_mark: