Change the source URLs to be like MXR's. Addresses bug 834509.

Also rename fetch_results() to results() for consistency with direct_result().
Make search URLs like /tree/search?q= rather than /search?tree=x&q=y. Closes bug 834509.

Invalid trees now return 404, because that makes sense now that we're putting the tree name in the URL path.
Change the search and source-code URLs to match MXR's. Closes bug 834509.