Added discussion comments avatars to diff

What does this MR do?

Adds the discussion note authors avatars into the sidebar of the diff. It shows the first 3 & then a count of how many more there is (if there are more).

By default only 1 is shown, then hovering over them opens the others out. When you hover over an avatar it will show a truncated version of the comment.

Clicking on an avatar will either open the discussion thread or close it.

Screenshots (if relevant)

What are the relevant issue numbers?

Closes #17237 (closed)