Add button to TOC theme component

[tvavrda (Tomas Vavrda)]

Also it is not intuitive. I told my colleague who is new to Discourse to pin a topic. He said there was no button. So I walked to him to show and well, no button, just a TOC. I was scrolling up and down and the wrench showed up briefly and then disappeared. So I was again scrolling randomly until I hit the first reply and we realized how it worked. It was not at all obvious. Especially on longer topics (which are exactly those to have TOC) it might be a challenge.