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.
In that video Till *specifically* requests the ability to edit all the same metadata as for the other attachments, including "ISBN, unique reference, and so on". These make as much sense for external attachments as they do for HTML ones, where the same form is shown.

In any case, all of these fields *are* displayed on the front end for external attachments, just as they are for the other types. I don't see why any of them are unnecessary.

I'll validate with the content editors but given both what Till says and what Paula previously said when I talked it over with her originally I don't expect this to need to change.