Fix comment truncation

Turns out that for the 'Show more replies' function to work correctly( as per here: https://github.com/guardian/frontend/pull/6231), we need to pass a parameter 'display threaded' to the the discussion API. This does this, reinstates the functionality to truncate reply threads and restyles the 'show more button' ( see below )

![more-comments](https://cloud.githubusercontent.com/assets/986155/4493988/eabee43a-4a4d-11e4-8e23-850cbe474bb5.png)