Image details license should fallback to dataset license

[msmolens]

An image's license is displayed in the image details pane on the image gallery. When the image itself has no license recorded, the image's dataset's license should be displayed. Currently the license defaults to CC-0 in all cases.

Reference: https://github.com/ImageMarkup/isic-archive/blob/49ca48b6c24d2ebf66fc1d303b6f38abb16801f2/web_external/ImagesGallery/Detail/ImageDetailsPane.js#L39-L45