Select and publish related contacts

When editing a contact, publishers can now select related contacts from a list; these get sent to the content store by their content IDs, which the content store will be able to look up and display with their current URLs.

This supersedes #161, as it includes the same commit as 973e6d0.

If this gets deployed before alphagov/content-store#51, the content store will reject the unrecognised `links` field.
Nothing here, it's only used on the associations on the model.  I've cleaned it up in 3989662.