[WIP] Do not show parents on the standalone annotation

When an annotation is loaded we now only show the annotation and it's children. We do this by updating the `scope.threading.root` property in the `AnnotationViewerController`.

This is one potential solution to this problem, which was introduced with changes to the Threading plugin in 847fba9. To me this fix makes sense as we want the primary annotation referenced by the url to be rendered as a card not as a reply in a tree.
What I can't work out is how the `threading.root` property changes, I can't find any code that assigns the variable other than the initialisation in the constructor.