D expand

Changes
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- mini refactor of comments.js (moving away from `Component` magic™)
- added expand/collapse thread button

![image](https://cloud.githubusercontent.com/assets/960919/4558115/a326c3f8-4edb-11e4-819e-d1fba42c6661.png)

![image](http://im.ezgif.com/tmp/gif_rev_1412766402_bbc23274a6.gif)
imo doing this stuff in js instead of html bloats the codebase, increases size of our javascript (which is served to all pageviews) and is much less readable and maintainable. let's talk about it in person though...