In the V2 API, the GET response for an individual notification
returns a 'cost' value, which we can get by multiplying the
billable units by the per-message rate of the supplier who
sent the message.
Any notifications with billable units > 0 but without a
corresponding `ProviderRates` entry will blow up the application,
so make sure you've got one.
This is used to construct a notification from the sorts of data an API call provides. This is used in both the db-email / db-sms tasks and the notifications rest endpoint to construct the notification DB object.