This change adds a uniqueness constraint on user addresses to prevent
duplicates.  A few changes follow:

- The user address forms in account and checkout are modified to take a
  user as a constructor arg so uniqueness can be checked.

- The assignment of a user when creating a new address has been moved
  into the form.

OSCAR_REQUIRED_ADDRESS_FIELDS allows setting which fields are required.
The use case is that post codes are expected to be required, but many
countries don't actually use them.

tests