Index: Make the buttons clickable.

Very ugly code at the moment because code is duplicated but it works. I'll make another PR that will make use of code reuse.

Screencast: http://gfycat.com/CircularShadowyBluejay
I think you could just use `{{this.topic.id}}` here, setting the data-topic attribute as `ia_topic-{{this.topic.id}}` and then incorporate the click event on topic buttons into the already existing event for topic filtering from the right pane.

What do you think?