[theme-reviewers] Themes directory upgrade

[Otto]

Okay, the theme downloads and the popular themes list download
counters should both be fixed now.

You may or may not see the counter fix for some time, as it's cached
for performance. So it may take a while for the cache to clear out.

Downloads was a header issue (new bbPress was sending 404s on those
links by mistake), so the fix to this should work immediately for all.

-Otto