When reviewing a diff of a file attachment, we store some data on the

attachment being diffed against. However, we weren't using this data

when showing thumbnails of comments in the review dialog or in reviews.
Before, a ReviewUI would be created that pointed to the attachment being

commented on, but without the diff information, leaving the ReviewUI to

have to grab it itself from the comment. To fix this, we now have a

review_ui property on FileAttachmentComment that builds its own ReviewUI

with the diff information.
Now, both the review dialog and reviews show the revision ranges for the

attachments, and both set things up to use an appropriate thumbnail. The

reviews page also reduces queries for diffed file attachments by

re-associating attachments, like we do elsewhere.

Tested with older file attachments, with newer attachments (single revision),

and with diffs of file attachments.
Note that in the screenshots, there are differences between the presentation and

style of both thumbnails. This needs to be dealt with still, but is a separate

issue.