Unable to relocate an existing Reference (right-click on panorama)

When such a Reference already exists but is not at the right (x, y) location, using the contextual menu to re-add it results in an IntegrityError as DRF tries to create a new Reference with the same composite key (refpoint, panorama).

The contextual menu should know of the ref that are already set on the panorama, and display an Update button instead of Insert, and use a PUT instead of a POST.