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