Remove ballot image file copying

[ranweiler]

Previously, we saved ballot metadata, and also made an application-local copy of any uploaded images (first manually selected, then indicated via manifest).

Now, instead, we are going to rely on the images existing on the filesystem at the absolute paths in the manifest.

• Remove code that attempts to copy ballot images
• Remove the file_path/src_path distinction in the data model
• Update frontend to indicate that there is only one meaningful path per ballot image file

[nealmcb]

I doubt the images will be at the same place. We might just be able to define a new root for the directory tree of images, but they may also be organized differently.