Update schema for recent changes to Popolo

There are a few new properties in Popolo which you may choose not to add, but I added them for now:

* Person#national_identity
* Organization#image
* Membership#label
* Membership#on_behalf_of_id

A few questions:

* Shouldn't `person_id` have type `"string"` instead of `["string", "null"]` on the Membership model?
* Shouldn't `sort_name` have type `["string", "null"]` instead of `"string"` on the Person model?
* What is the reasoning for having an `organization_id` property on each Post? Won't it always be equal to the embedding organization's ID?

Besides that, I updated a few property descriptions to match Popolo.