Require Sign in class

# Why
Trying to make our discussion pages as cachable and performant as possible. User data and state makes this a little tricky. We're shifting a bit of the responsibility to the CSS, which might seem a bit odd, but is by far the tidiest. [See here for more explanation](https://github.com/guardian/frontend/pull/2384).

This is our first attempt of not cluttering up the JS with the reply button.

There is another small edge case which has been made easier with this is read only mode.

![Faster!](http://gifs.gifbin.com/102011/reverse-1319482109_fast_shirt_removal.gif)