Add field for all images associated with a document

[jduss4]

Right now, image_id is used for the preview / thumbnail view of a document, but there is currently no field for all the images. multivalued, keyword field would be best. Will need to research to determine which dublin core / etc metadata schema we will loosely imitate.

[jduss4]

Ideally this would describe the images, resource location, and method of retrieving them.

For example, something like

images: [
  {
    id: "nei.001.001.jpg",
    endpoint: "mediaserver.unl.edu/neihardt",
    protocol: "http://iiif.io/api/image/2/context.json"
  }
]

(something along those lines, meant to be pseudo code)

That way we know that we need to construct a URL with the IIIF protocol, not just putting the image filename on the end of the location.

[karindalziel]

I wonder if we need the endpoint and protocol as part of the nested field, since they will likely all be the same for any given API item. perhaps instead those should be separate fields in the API, and the images should simply be a list of image ID's.

We will likely need to do a proof of concept for this for the whitman EAD files/integrated guide