Proof email notifications - add a date/time stamp to new comments

[#1]

Currently, proof email notifications add new comments for the entire comment thread, but there is no way to tell which of the comments are actually new. A date and time stamp to each comment would be helpful when a user is skimming through the email for what is actually new.

In a proof, they have the date/time stamp for each comment.

But when the email notification comes through, there is no way to tell what is new and what has been added to.