Update Pombola's Person and Organisation models for better Popolo compatibility

This largely fixes #1338. There are two missing parts, neither of which
are used in the current `pombola/south_africa/data/south-africa-popolo.json`,
so it's probably OK to postpone adding for the moment:

* Adding a `Post` model
* Adding a `Link` model, with many-to-one relationships to Person and Organisation