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)
duplicate `data-test-id`?