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.
if [my concern](https://github.com/alphagov/whitehall/pull/1776#discussion_r18879331) about external attachments not needing most fields is valid, then i'd suggest we re-think whether `ExternalAttachment` should be part of the `Attachment` STI. the valid use case for an STI is where inherited classes have similar attributes, and differ in behaviour. in this case `ExternalAttachment` records will end up having a lot of empty fields.