This change does a few small things to polish up the next/previous file

attachments feature:

Instead of just relying on FileAttachment.__str__, we use the

display_name property for the caption. This will fall back on the filename

  if no caption has been set.
The style for the next/previous switchers had a huge font size set, which

  looked kind of goofy and didn't match what we had in the review request box.
We have a "dummy" file review UI for file attachments that aren't reviewable.

  This had some code to set the caption in the review UI header in the case

  where we were looking at a diff, but didn't set the caption when there was

  only a single revision.

This dummy review UI is still pretty dumb, and should probably have some

  indication that the file can't be displayed inline, along with a download

  link, but that can come in some later change.

Switched around file attachments and saw that things looked better.