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.
I _think_ these two lines can be combined: 

`validates :external_url, uri: true, allow_blank: true, presence: { if: :external? }`