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.

Also I moved the generate_hash fn up to the Address superclass as it's
possible that it might be used in other Address classes.