Infinite Scroll: Add a class name or identifer to the IS button

As suggested by @davidakennedy in #1205 we may want to add a class name or an identifier to the button that triggers Infinite Scroll:

> One suggestion I would make is maybe adding a class name to the button. Something like: infinite-trigger. That would encourage developers to target the class name in the future, instead of specific elements. It would free us up too, since if we ever had to adjust the elements, we could still keep class names the same and keep breaking things to a minimum.

Start with the Infinite Scroll button implementation and review the suggestion in issue #1205. Add a stable class name or identifier to the button that triggers Infinite Scroll, then verify that the rendered button exposes it for developers to target.

Written by the indexing model from the issue text.