Add property for numeric address code

This probably should be done with a migration to change the field type
of the existing numeric field, and to correct all values to use the
3-digit form.  However, that would be quite painful, so I have
shamefully opted for the easy way out: provide a simple class property
to format the number in the right way.
Revert review form API to be more explicit

See https://gist.github.com/codeinthehole/6178034 for some discussion.