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.

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

Reviewed at https://reviews.reviewboard.org/r/4829/