Search for artwork images by license

[lklic]

Currently we have no image license info in the metadata but this will be added soon.

For now, I am adding a placeholder issue with the request to implement a structured search for images based on license.

[PietroLiuzzo]

Information needs to be added to the data, as it is not transformed at the moment. information needed also for #436

[PietroLiuzzo]

licence should be added to the mapping and to the solr indexes

[PietroLiuzzo]

136