Fix serving uploaded media on a dev server.

The recent fixes for static media resulted in /media/ serving up static
media files instead of the /media/ directory. This happened because the
document_root was no longer set and the static file server was used.
This reverts that part of the code.
Fix the interdiff selector.

The diff-context resource was broken because of Christian's local site name
refactor. Because this is now processed in a decorator, the local_site_name
needs to be the first parameter instead of the last.

Testing done: Used the interdiff selector successfully.

Reviewed at https://reviews.reviewboard.org/r/5440/