Discussion ordering ui

# Open: Discussion - comment ordering UI

Based [https://github.com/guardian/frontend/pull/2680](on this), 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.

## Notes
Some of the pull request [https://github.com/guardian/frontend/pull/2680](is here) - once this is done, the PR will be a little cleaner.