[#3788] Fix LabelListingField.to_representation to work in DRF UI
[#3788] Fix LabelListingField.to_representation to work in DRF UI

To display the form in the Django Rest Framework UI, the serializer is
passed individual items in a queryset, instead of the whole queryset. This
commit fixes the serializer to be able to handle individual instances in the
queryset correctly.

akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr,akvo/akvo-rsr