[Reviewed] Direct uploads to S3

Closes #541.


This only covers submission files, is there anything else that needs to go to S3?

Multiple uploads work okay. Below the 'Choose file' button you'll see the names of the files when the upload finishes.

If a file gets uploaded but the form doesn't get submitted (someone presses 'Cancel', or whatever mishap), the file still gets stored on S3. So we may want a rake task to periodically delete any files that Rails doesn't know about.