- User prefetch_related to load data ahead of time
- Adds a new simple serializer, that does not use properties that
  triggers new queries

Introduces the invalid field on the Identity model. The field is used to
indicate that the Identity is in invalid. This is useful when a value is
not a working value, even though the value passes validation checks,
like unused national id numbers, emails tied to non-existing domains,
etc.

Resolves: GREG-252

Move serializers only used by gregui.

This ensures that identity values created from values written in the
frontend by guests are given a source value signifying so.

Resolves: GREG-186

Remove the 'email' and 'mobile_phone' fields on Person and use identities to represent this data instead.

- Migrate data on existing Person objects to Identity objects
- Make the 'is verified' logic only accept certain ID types
- Sync the Person.is_verified and API 'verified' filter logic