Show a configurable message to customers when Affirm is selected in checkout

In fact, instead of using a simple textarea, let's make this a text_format to permit the use of WYSIWYG in the message preparation if desired.

I've implemented this as described and included a container div in the checkout form to make it easier for specific targeting, as the payment method add form doesn't actually wrap everything in its own container.

Settings form:

Front-end instructions:

The only oddity in the current approach is that we don't have weights on elements of this form, so it's hard to position things appropriately. I'll ask jsacksick re: revising the checkout pane in core to at least ship some default weights.

Reviewed by jsacksick, committing. Will also open relevant Commerce Core issues.