Add comment thumbnail rendering for file attachments.

This gives file attachments the ability to render the contextual
thumbnail for a comment. By default, there's no thumbnail, but a
ReviewUI can provide a get_comment_thumbnail function that returns HTML
representing the comment.

So far, only ImageReviewUI implement this. Comments made on image files
now show the region commented on, just like screenshots.

Viewed some comments I already had on image file attachments. Saw that
the commented area appeared as a preview.