Add a Write.as API endpoint that accepts generic files and stores them in an Amazon S3 bucket.
This supports the #fight_back_news migration.
This should return a JSON object very similar to (if not the exact same as) the Snap.as API.
- Objects should keep their original filename
- If an object with the same filename already exists, append an incremented number or random string to the name of the new object
- Objects should be stored with an attachment content disposition, so viewing them automatically starts a download