Unanswered filter button doesn't 'light up'

[awesomerobot (Kris)]

I’ve got a PR ready in our review queue that refactors this into a dropdown:

https://github.com/discourse/discourse-unanswered-filter/pull/1

This will rely on a minor update to Discourse as well (for the “answered” filter):

https://github.com/discourse/discourse/pull/16042