Clean up extension reorganizing merge regressions

Close #1361
Ensure the vendorized theme css is copied over

With a debug build of the extension, these were not processed at all
and not otherwise copied. Rather than add copy commands for each in
the build script, it seems rather nice to just smush them together
and tell webassets to ignore debug for their bundle.
Copy vendor libs into bundled extension

It's not necessary to do this for production builds, because they
get minified into the target directory anyway.
Appease newer webassets in extension build