Fix display of standalone messages

This is an alternative solution to #1803. Sadly it's not as clean a fix but at least this one actually works.

In the AnnotationViewController we now set the `shouldRemoveEmptyParents` flag to `true`. When `pruneEmpties` is called this then removes any empty parent annotations and promotes the child up the tree giving us the behaviour we want.

Now when you view a standalone annotation page the annotation referenced by the url will be displayed in the top level card.