Improved logic of showing comments

The logic of this kept throwing us into weird-whathappening-land.
Moved the logic over to the server and a wee bit of SasS.
This also lightens the load of [`discussionFooter`](https://github.com/guardian/frontend/blob/master/common/app/views/fragments/discussionFooter.scala.html) which we will be doing more of.

![Clean up](http://a.dilcdn.com/bl/wp-content/uploads/sites/2/2013/05/GIF-FANTASIA-BROOM2.gif)