Add proxy for programming image URLs

We want some sort of permanent URL for images, and this is one way to do that- we store them on S3 and have varnish alias a kasandbox.org URL to them.

This is easy for me to maintain and add new images to without affecting webapp deploys.

Discussed in:
https://docs.google.com/a/khanacademy.org/document/d/1e4ksRVLZOS3nKP2SR-dTSw1lXlkwZl0Eq0TG6HKrvuI/edit?disco=AAAAAMgVOGk