New Proof Permission - Can Reply But Not Make New Comments

[#1]

For most of us, we'd like to lock a stage after a decision is made so that new comments are not entered as the proof moves through its stages. However, there are a lot of cases when someone replies to a comment with a question (or it is clear they didn't understand the comment) and someone in a locked stage needs to reply back.

Would like a new stage setting of "No New Comments" which is like Locked but allows them to reply to comments.