Add a hook for working with comments in the review dialog.
This new hook, ReviewDialogCommentHook, allows an extension to display

data or provide UI for a comment in the review dialog. It takes a view

type that will later be instantiated and passed the comment as a model.

The view can do whatever it needs to with the model, including providing

UI to set new data on it.

Tested with an extension I'm developing. I was able to see the current

state that I had set in extra_data, and modify that state and successfully

save it.