This change does a lot of polishing for the new file attachment revisions

feature. Notably, I was highly dissatisfied with the headers above the files,

which looked cluttered and unaligned. I've moved everything into a single

header box (with a slight gradient), and tried to align things as best as I

could. I've also simplified what gets shown, since we really don't need to say

everything three or more times.
While I was doing this I made a couple small cleanup changes to the code to be

a little clearer.

Created several file attachments with images, text, markdown, and other

  files. Tested that things looked good when there was just a single revision

  (showing just the caption and the file), and when there were multiple

  revisions (looking at either a single one of them, or showing a

  diff/comparison where appropriate).
Ran unit tests.
Ran js-tests.