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 don't think most of these fields make sense for an external attachment. if i understand correctly from [Till's description of their needs](https://docs.google.com/a/digital.cabinet-office.gov.uk/file/d/0B3k4KdITtbEOVHJxelJaSG9jbmc/edit), we need an attachment that has an external url, editable title, and whose order can be changed. an additional 'should have' that came up is 'Unique reference' field, because he suggests that it's better to have a field for it instead of appending it in the title, which they almost always do. apart from these, Till doesn't very clearly state whether the other fields are needed or not. hence, we must cross-check with them which fields are necessary.

we should not be showing any unnecessary fields to the editors, because:
- it may seem to suggest that they're supposed to be filled for external attachments
- they're not used or surfaced anywhere
- we run the risk of collecting worthless data