Make API consistent between resources and gallery items

[mcarans]

In HDX the relationship between datasets and resources vs datasets and gallery items is the same ie. a dataset can have one of more resources or gallery items. Resources and gallery items must be associated with one dataset. This is not reflected in the API which follows CKAN related items for gallery items.

Either the gallery item should be like a resource ie. downloaded/uploaded as part of dataset (package_create etc.) or the resource should be more like a gallery item ie. not part of the dataset. It is confusing having two different ways of handling these HDX objects.