[wp-forums] Removal of /extend

[Otto]

As many of you may have noticed, /extend/plugins, themes, and ideas,
all now have top-level names. Extend is gone, long live the top-level!
One namespace to rule them all!

Anyway, this has caused a number of minor issues everywhere. We'll fix
them as they crop up. Forward any problems you find to us and we'll
take care of them.

- The /extend/themes/foo redirect issue should have been fixed last
night, but some people might still see cached redirects. This will
self-heal eventually.
- The Theme downloads via built-in WP installer should be working
again. Feel free to #blamenacin for forgetting to commit those API
changes last night. :)

-Otto