greg.models: fix PersonIdentities link to Person

A Person may have multiple associated PersonIdentities.  The object
relation link between these data structures was previously introduced
as "person", but is renamed "identities" with this patch.

This changes the access pattern from:

	person.person.all() -> QuerySet[PersonIdentity]

To:

	person.identities.all() -> QuerySet[PersonIdentity]

Fixes: GREG-20