Avoid assigning notification.template when creating test objects

`Notification.template` changed from being a Template relationship
to TemplateHistory. When a relationship attribute is being assigned,
SQLAlchemy checks that the assigned value type matches the relationship
type. Since most tests at the moment create a notification using a
Template instance this check fails.

Rewriting the tests to use TemplateHistory objects would require
changes to the majority of tests. Instead, when creating a notification
objects we assign the foreign key attributes directly. This skips the
SQLAlchemy type check, but we still get the constraint check on the
foreign keys, so a matching TemplateHistory object needs to exist in
the database.

tests/app/conftest.py

@@ -544,8 +544,7 @@ def sample_notification(
         'job': job,
         'service_id': service.id,
         'service': service,
-        'template_id': template.id if template else None,
-        'template': template,
+        'template_id': template.id,
         'template_version': template.version,
         'status': status,
         'reference': reference,
@@ -624,7 +623,7 @@ def sample_email_notification(notify_db, notify_db_session):
         'job': job,
         'service_id': service.id,
         'service': service,
-        'template': template,
+        'template_id': template.id,
         'template_version': template.version,
         'status': 'created',
         'reference': None,