`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.
we now no longer create a job. At the end of the post there is no
action, as we don't have any tasks to queue immediately - if it's a
real notification it'll get picked up in the evening scheduled task.
If it's a test notification, we create it with an initial status of
sending so that we can be sure it'll never get picked up - and then we
trigger the update-letter-notifications-to-sent-to-dvla task to sent
the sent-at/by.
