Add a hook for displaying extra comment information for reviews.
This introduces a new hook, CommentDetailDisplayHook, which can be used

to display additional data in reviews and in e-mails (both HTML and

plain text). An extension can use this to show information such as, say,

the severity of a comment.
This does not provide the ability to display in the review dialog, as

that's generated in JavaScript entirely. That will be added in a future

change.

Tested with and without rendered content in the reviews, HTML e-mails,

and plain-text e-mails. Everything looked as I expected it to look.