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.
could be changed to: `contact.related_contacts << FactoryGirl.create_list(:contact, 2)` to not repeat and save a few μs.