Add a link to the IIIF manifest

[thatbudakguy]

If there is a link to the manifest at references key http://iiif.io/api/presentation#manifest, render a link in the sidebar with text "IIIF Manifest" or something like that. Could also throw in the IIIF icon if we wanted to.

[thatbudakguy]

For the image reference only (info.json URL), is it even useful to have a link? Not sure.