Improve the look and feel of the file attachment thumbnails.

The file attachment thumbnails were looking a bit "busy." This attempts
to simplify it and make things more usable in a few ways:

1) Thumbnails are now of a fixed size, so that they nicely fit into a
   grid.

2) The borders have been lightened, and the background for the caption
   has been removed. Instead of the background, there's a very faint
   border above it, in order to provide the simplest of separation.

3) The red "X" for deleting a file attachment has been removed. Instead,
   a black-and-white trashcan icon has been added as an action item
   alongside "Review."

4) Any part of a filename that gets cut off will be ellipsized on modern
   browsers (or just hidden on older ones).

5) The pencil icon is now on the right of the caption area, instead of
   right up against the text.

6) The edit field for the caption now spans the width of the caption
   area, and doesn't have an ugly focus border around it.

7) A new tooltip appears over the thumbnail overlay saying "Click to
   review."

This also renames the "File Attachments" section to "Files," as files
are going to be more important in the future, and "attachments" makes it
feel less first-class.