Make submissions editable -- resubmitting will now change what appears in the gallery
 * shows only the last submission
 * Attachment model / attachments table added
 * app/services/canvas/attachments_processor.rb added
 * also don't do many actions if there is no change
Merge pull request #59 from coyote/edit_submissions

Make submissions editable -- resubmitting will now change what appears i...