Mediatype "image" can have multiple images

[hadro]

See this item, for example:

• Item page: https://archive.org/details/arkivkopia.se-lms-G70-48.3
• Metadata: https://archive.org/metadata/arkivkopia.se-lms-G70-48.3

The logic in the app resolve right now only takes the last image in the file list and creates a manifest for that single image.
We'll need a way to check for multiple valid images, and then add those as canvases individually.

[glenrobson]

Transfering to new repo: internetarchive/iiif#52