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/
Fix the API docs generator.

The webapi docs build process was still using the deprecated
allowed_item_mimetypes and allowed_list_mimetypes fields. Update it to use the
consolidated allowed_mimetypes field.

Testing done: Built the API docs.
Reviewed at https://reviews.reviewboard.org/r/4832/
Clarify in the docs that you need to upload full diffs.

When updating an existing review request, some people try to upload just the
incremental diff from their previous change, which doesn't work. Clarify in the
docs how to do this.

Testing done: Built HTML docs.
Reviewed at https://reviews.reviewboard.org/r/4831/
Fixes bug 2899.
Reduce database queries when returning the User list resource.

The User API resource checked profile.is_private on each User being
serialized in a list, which is expensive. Unfortunately, neither
select_related() nor prefetch_related() appear to properly cache reverse
ForeignKey relations, so they don't help us with this.

The best solution, it seems, is to tack on an extra query for our users,
storing the result of Profile.is_private on the User. The
is_profile_visible function checks for this, and attempts to do the
right thing by making use of that value, if it's available.