This change dramatically improves the markup and CSS for review threads.

Specifically, instead of using <dl> for everything (which isn't semantically

correct), we wrap things in <ol> elements, and then individual comments are

broken up into <div>s with sensible classes. These are then styled with

consistent and simple spacing so that everything looks nice while also being

aligned. Replies to a comment are all indented a bit from that comment,

emphasizing the relationship in the way that most threaded displays work. One

particularly nice part of this change is that the text for comments is now

horizontally aligned with the avatar (the borders of the text area are

accounted for using negative margins).

Tested reviews with all sorts of different comments, and replies to those

reviews. Checked to make sure everything was aligned and spaced correctly.

Opened the editor for draft replies to verify that there were no jumps in the

position of the text.