Motivation: It seems there is not always a lot of clarity about the areas of concerns of the yaml files in processors' config directories in DonationInterface. Specifically, some parts of the config seem only front-end-y, and others seem to control both back-end-y stuff (that could go in SmashPig, like what subpayment methods a providers supports) and font-end-y stuff (like which options can be shown to the user).

Let's review how all this works, document what is not documented, and consider possible improvements. It feels like some things configured using these files in DonationInterface would be better set in SmashPig config, with clear mechanisms in DI regarding how a processor's setup determines what is shown on a form. We could then separate out the config bits that are only related to the front-end.

Old notes on this topic: Refactor 2015