Support images and image versions

  * Use CarrierWave to manage images
  * An image record represents the original image
  * An image can have multiple image versions for varying image sizes.  It
    currently only supports small image versions.
  * Deleting an image will delete all its image versions
  * Default to not available image when image record exists but its file is
    missing
  * CarrierWave will store images on S3 in production environments, otherwise it
    will store images locally. Image uploads created by tests will be
    automatically removed.

Update ItemsController

  * Use ItemPublicData when rendering both index and show
  * Add support for rendering collection of items as ItemPublicData
  * Fix search header

Add seed data for development

  * Include original and small versions of stock images

Remove unecessary test setup