Read UI identities from replica database

[emyller]

Endpoint: /api/v1/environments/<environment_api_key>/identities/

Since #6009, this endpoint consumes identities from the primary database. It should consume from replicas when available.

Need to use using_database_replica in the Django queryset.

[matthewelwell]

Given the infrequent use of this endpoint compared to the SDK endpoints, I don't think this is worth the investment of time.