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 @vinayvinay  has a valid point in that we could (and in an ideal world, probably should) have attempted to implement this without STI. But given that `FileAttachment` and `HtmlAtachment` already use it and that the work required to migration from it would have been [extremely involved](https://about.futurelearn.com/blog/refactoring-rails-sti/), On balance sticking with the existing pattern for attachments is the least painful option in the short to medium term. Longer term, we should almost certainly avoid using STI in the new tool that eventually replaces much of whitehall.