Reintroduce hidden blank field for contact groups.

Without this field, it's not possible to remove all contact groups from
a contact, because the browser doesn't send any values if nothing is
selected in the form control, which Rails takes to mean "don't change
the value of this field".
Merge pull request #159 from alphagov/fix-cannot-deselect-groups

Reintroduce hidden blank field for contact groups
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.