When the S3 store was enabled, we were only applying the S3 CDN.
So all images stored locally, like the emojis, were never put on the local CDN.
Fixed a bunch of CookedPostProcessor test by adding a call to 'optimize_urls'
in order to get final URLs.
I also removed the unnecessary PrettyText.add_s3_cdn method since this is already
handled in the CookedPostProcessor.