External attachments

https://www.agileplannerapp.com/boards/173808/cards/5995

Replace the external_url functionality in Publications and Consultations with a new ExternalAttachment type. This displays along with the other attachments (rather than a separate 'held on another website' link) and allows editors to edit the title and reorder them. The data migration creates external attachments in the first position.
@tekin Done in bf522fc