Add relationships between contacts.

Relatedness between contacts is asymmetric (just because A is related to
B, doesn't mean B is related to A) and currently has no inverse
relationship defined; we can add one if and when we need it.
Add related contacts to the contact form.

For now, we can get away with using selecting all the contacts and
dumping them into a multi-select field, because there's only around 100
of them. If and when we get to the point of using this for more contact
details from around government, we're going to need to do something a
little smarter with this field, because:

  * the page is going to get slower and slower as we add contacts
  * it's going to get harder to find the contact you're looking for

This should be good enough for now, though.
Send content IDs to the content store.
Send related contacts' content IDs as links.

The content store will be able to look these up by their content IDs and
expose their current titles and URLs to the frontend for rendering.
Make contact_relationships foreign_key columns 'NOT NULL'

Also tidy up the migration a bit, and remove the `class_name` param that
does nothing and is ignored.
Merge pull request #162 from alphagov/related-contacts

Select and publish related contacts