Remove NotificationHistory.template relationship

Relationship attribute is not used by the application code, so we
can remove it without replacing it with a TemplateHistory one.

This also updates the foreign key constraint to refer to the composite
primary key for the TemplateHistory records.

tests/app/conftest.py

@@ -679,7 +679,7 @@ def sample_notification_history(
     notification_history = NotificationHistory(
         id=uuid.uuid4(),
         service=sample_template.service,
-        template=sample_template,
+        template_id=sample_template.id,
         template_version=sample_template.version,
         status=status,
         created_at=created_at,

tests/app/dao/test_provider_statistics_dao.py

@@ -73,7 +73,7 @@ def noti_hist(notify_db, template, status='delivered', billable_units=None, key_
     notification_history = NotificationHistory(
         id=uuid.uuid4(),
         service=template.service,
-        template=template,
+        template_id=template.id,
         template_version=template.version,
         status=status,
         created_at=datetime.utcnow(),