Mutating dictionaries is gross and doesn’t work as you’d expect. Better
to have the function return a new dictionary instead.
Means we can be explicit that `created_by` is one of the allowed params
when updating a service.
We can filter all notifications by status already. This commit reuses
the same code to filter the notifications for a job by status.
This means that, visually we can show the count on a job the same as
we do for all notifications, which is similar to how we show the counts
on the dashboard, so hopefully it feels like a bit more of a solid
thing.
This also applies to CSV downloads and AJAX updates, which will inherit
any filtering that their parent page has applied.
