"Fix" images

Summary:
Allows hotlinking images uploaded to aws.
Disallows hotlinking other images.
(It doesn't check whether it's our bucket, but)

Test Plan:
https://ka-perseus-graphie.s3.amazonaws.com/2d85ebc9f66db5948e4e163860d824b6707bd6d4.js
https://ka-perseus-graphie.s3.amazonaws.com/bea695a5e1eb2f04a47e694c189523b4a8f936cd.js

Reviewers: eater, alpert

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D12761