Have CreditCardAccount.need_reconciling return an ActiveRecord::Relation instead of an Array

This should resolve the recent spec failures.

Initially I wrote a smaller patch that changed the code to expect an `Array` and left `need_reconciling` alone*, but it feels better to me if it returns a relation.

* https://github.com/tablexi/nucore-open/compare/selected-account-updates?expand=1 or 59719fd2d6816e7a2c97e9df730a9be747c4b19d...5b3bd720c9cea4b20801bb1146c415531b4e3841
I'm in full support of sharing this code somewhere, but I'm not sure AffiliateAccount is the right place for it. Perhaps we make a `ReconcilableAccount` concern? Or we could pull it all the way up to Account, and then if NufsAccount were to need to override it (it doesn't look like we need it), it could.