Fix external account schema

It is valid for there to be an empty collection returned.
push to card: fixture changes
require fields needed for push to card
push to card scenarios
make card.category an enum
fix match syntax
return fields relevant to push to card in the card token
fix format of json body match
scenarios to produce different statuses for push to card credits. closes #587
Fix my own complaints.
map "charge card" => "credit" to avoid confusion
explanatory text for the credit scenario
Add second failure scenario
Add special error codes for the two failures.
re-use existing category code
Let's verify that credits get created.

There's a side effect that we weren't checking for before, so let's do that.
debits -> credits
remove push to card scenarios from cards.feature and credits.feature
most push to card scenarios to push_to_card.feature
remove \n's from error schema