Image label import

[mzur]

Implement an image label import similar to the image metadata import. Users can upload a CSV (or Excel?) file that contains labels for each image (identified by filename). The labels could either have the same name than labels of a specified label tree, be identified by their ID or be automatically created in the specified label tree during the import.

[mzur]

This could be done with an iFDO import. At some point, we might support the import of annotations through iFDOs. The import of image labels would be the same.