Tweak and update the admin UI style.

The admin UI style has been updated to match the new Review Board UI
style. It has the new header bar (though in a yellowish color to signify
that it's the admin UI) and bits of the UI match the RB color scheme and
images.

The admin login page has been replaced with our standard login page.
This reduces complexity quite a bit.

Rather than fight the Django admin CSS files, we now ship our own
version of their admin.css. While we may have to update it as Django
updates theirs, it's overall less painful.

Tested each page of the admin UI that I could think of that would be
affected, in both Chrome and Firefox.