Revising editions loses topical featurings

Tracker: https://www.pivotaltracker.com/story/show/48500995

Classfication featurings are copied onto newly drafted editions but because of the way file upload quarantining works, the image file will not be where carrierwave expects it to be, causing the "presence" check on the file to fail in the ClassificationFeaturingImageData instance. The end result is that any copied ClassificationFeaturings will not be valid (and thus will not save), causing the new edition to lose any featurings on topical events it may have had. By only validating the file when it changes (i.e. when it's initially created or when a new file is uploaded), classification featurings will be correctly persisted across newly drafted editions.