AM buttons concept (CSS portion only)

DO NOT MERGE!

This is a concept for discussion of Attribute Module CSS (http://glenmaddern.com/articles/introducing-am-css) on the fireplace buttons. The comment at the top of the file describes its use in markup, while the JS toggling of attribute values would be messier since we won't be able to enjoy `.classList` etc.

Implementing this across the project would be prone to regressions but it was worthwhile thinking about how it would change our UI development moving forward. We would need to start with interim refactoring like `.loadmore [ui-button] {...}` that would eventually become `[ui-wrapper~="loadmore"]` etc.