Discussion ordering ui

# Open: Discussion - comment ordering UI

Based [on this](https://github.com/guardian/frontend/pull/2680), we now allow users to do this:
![Order please!](https://f.cloud.github.com/assets/31692/1870583/cc266af2-7889-11e3-8474-387857c12f99.jpeg)

We have intentionally left the styling out to make it evident that people can do it.

## How this works
* If a user changes this, it is saved as a pref and trumps all
* If a discussion is open, we are newest first
* If a discussion is closed, we are oldest first

![Order please](https://f.cloud.github.com/assets/31692/1870646/8a78cc20-788a-11e3-9a27-b48ddb65a6cd.gif)