Issues/775 display constituency office relationship

Closes #775.

This puts a ZA specific code into the `core` app (`constituency_offices` onto the `core.models.Person` class and a conditional in the `person_base.html` template) which is not ideal, but I couldn't think of an easier light touch approach.

Also incidentally fixes some other template behaviour that tested `settings.COUNTRY_APP` in the templates, but as it was not exposed was always `None`.