[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.
@tilgovi I need this here to ensure that ctrl.container is always up-to-date with the current value of `Threading.root`. However if you open this branch and load the homepage you'll see the following error:

> Uncaught Error: [$rootScope:infdig] 10 $digest() iterations reached.

I'm assuming that something is going wrong with this watcher, but as far as I can tell it should just be performing a strict equality check on the object.