@craigspaeth: Switch out Showdown for Markdown.js

Showdown is no longer maintained so let's switch it out for something that is. I chose https://github.com/evilstreak/markdown-js (https://github.com/chjj/marked is another option though).

This just straight up disables HTML in Markdown as well. I don't believe we rely on that anywhere. Force's suite passes fine.

I replaced the `toText` stuff with underscore.string's `stripTags` implementation, for consistency. Neither is going to protect us from XSS, of course though.