Add ability to add images to a concept

[koenedaele]

One of the most requested features has been support for images to illustrate a concept (since 2014). However, the SKOS standard has no support for images. This will make it hard to import thesauri and export them. Although I don't know any thesaurus exporting images, so perhaps it's a purely Atramhasis feature.

A good solution might be linking to images, but not directly uploading them to Atramhasis. This could be very simple such as "provide a URI for an image file", but then we need to determine if we want any metadata on that image to be present as well. A more sensible option might be to allow linking one or more IIIF-manifests (https://iiif.io/) to a concept. This way Atramhasis does not need to become a full-blown DAM, but by including a manifest URI and a client side viewer, people are able to illustrate concepts.