FIX: thumbnailing isn't working with CDN enabled

FIX: image thumbnailing isn't working when CDN is enabled.

The issue was that by the time we'd run the `CookedPostCreator` job, we'd have already prepended the cdn (`PrettyText.apply_cdn`) to all the assets in the cooked post. So we need to remove any cdn before trying to retrieve the corresponding upload from the databse (where urls are stored relatively).

Also fixed/cleaned-up some tests.