Fix replies toggle visual glitch in edit mode

This fixes an issue (#1808) where the replies button overlaps the license message when the editor is switched into edit mode. 

This is achieved by only applying the negative margin on the `thread-reply` element when the annotation is in "view" mode.

An `on-mode-change` handler is now passed to the annotation directive via an attribute which is called when the view mode changes. A class applying the negative margin is then toggled on the reply element in the thread template.