Update varnish config to allow package-manifest to be proxied through kasandbox

Test Plan:
Reloaded in production, then loaded up
https://1218-0928-521cbbd43900-dot-khan-academy.appspot.com/computer-programming/new/pjs
(/computer-programming/new/pjs on any other deployed version) and no longer saw
a 403 accessing
https://1218-0928-521cbbd43900.kasandbox.org/package-manifest.bd9a80bbc96ba9a7a843f39dc0c04bd9.js?lang=en

Auditors: csilvers, alpert