Not all example groups should have the controller methods applied.

Stripped in solidus

Of course this flexible won't work everywhere, but calling bankruptcy on
guessing which versions are minimum for right now

Since this branch is to support Solidus, we probably want to be sending
along the Solidus version and not the Spree version. Spree.version is
removed as of Solidus 2.x and will actually cause errors.

In the JavaScript, it's better for us to rely on the name of the input to determine if the customer has chosen Affirm. Picking the first checked radio button will cause some issues with [the wallet code](https://github.com/solidusio/solidus/blob/master/frontend/app/views/spree/checkout/_payment.html.erb#L6) as well as anywhere that chooses a different method of recording the payment method ID.

Make payment method check more resilient

This commit into core:
solidusio/solidus@547d459

Added a check if payment sources are `#reusable?`. The
`SolidusAffirm::Checkout` model here is used as the payment source, but
doesn't implement that method, causing any code that looks up reusable
sources to throw a method missing error.

One example where I've seen this error is the /admin/payments/new page.

Make Affirm checkouts non-reusable

Some strings we print on _affirm.html.erb may contain quotes and must be escaped.

Escape javascript strings