Move popolo validation logic into mongoose models

[chrismytton]

Add explicit validations to the mongoose models rather than having the validation performed by json schemas. This is how django-popolo handles things.

This change would mean less magic going on with the validations, which are currently done in middleware. It would also make it easier to express things like "All memberships must assign values to either member, person, or person_id and to either organization, organization_id, post or post_id."

[akuckartz]

An aside: Validation of a JSON-LD serialisation could use RDFUnit.