Add content IDs to contacts

This is in preparation for sending related contacts to the [content store](https://github.com/alphagov/content-store).

To support relationships between content items that still work when the target moves from one URL to another, we’ve decided to add a unique, immutable ID to each content item that’s only visible within the publishing system itself; the content store will translate these into URLs for published content.
This has the same description as the test above.  Would be good to clarify how this differs...