Add image ID to export

[lgelauff]

Jurors see an image ID to identify the image without being biased by their title. We received an email from a jury member referring to that ID, but then had no way to figure out which image that ID belonged to.

It would be nice if the coordinator had a way to figure out easily which image that ID belongs to. One way could be to add the ID as a column in the export files (especially the 'download entries' file).