Image Gallery: Set Paperclip 2.4 paths for existing resources in PC 3.4

TOOD: revisit this, and write a script to move old attachments.

Meanwhile: image.rb doesn't need this, because there were no older attachments.
Image Gallery: Paperclip 3.4 needs aws-sdk, not aws-s3.
Image Gallery: Paperclip modifications.

Merge branch 'topic_np_image_gallery_from_itsisu' into topic_au_np_rational_images

* topic_np_image_gallery_from_itsisu:
  Image Gallery: Paperclip 3.4 needs aws-sdk, not aws-s3.
  Image Gallery: Set Paperclip 2.4 paths for existing resources in PC 3.4