Pull the old djblets sort images into the reviewboard tree.

The reviewboard admin UI uses the sort icons from djblets to indicate the sort.
Unfortunately, when I converted djblets to a spritesheet, this broke\. Rather
than try to rearchitect the admin tables to use the spritesheet, I've just
pulled the old images into the reviewboard tree. While I was doing this, I made
it use an icon for descending sorts in addition to ascending, and pulled the
header background image into the reviewboard tree as well.

Viewed the admin UI and set sort orders on a couple different columns\. Saw that
I could now see the icons.