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.
While that's true, the alternative would seem to be to make the code **massively** more complex for very little benefit. You'd need a separate model and controller, some way of keeping a unique ordering among unrelated types, a way of getting and displaying those unrelated types on the frontend; I just don't see why that is worthwhile.