added migration to generate curators by delayed job

This is a replacement for a migration that was failing before - https://github.com/theodi/open-data-certificate/blob/master/db/migrate/20130731203215_recompute_dataset_curator_for_certificates.rb - the task will be done as delayed jobs, so shouldn't chew up resources on deploy.

Once completed - the publisher will be displayed on https://certificates.theodi.org/datasets