Update for the changes to djblets static media.

This pulls in the stuff required from the djblets static media move. Part of
this means that we'll re-ship the djblets media as part of the Review Board
package, but that's fine.

Testing done:
* Verified that pages like the dashboard and extension list still worked and
  looked correct.
* Ran build-media.py and checked the results.

Reviewed at https://reviews.reviewboard.org/r/4655/