This will allow admin to pass through a value of "government" for the
broadcast_channel. We don't have any logic around the value of service.broadcast_channel,
so no updates are needed to the tasks etc.
We want to replace the value `None` for
service.allowed_broadcast_provider with the value of "all". As a first
step, we need to allow both values. Once notifications-admin has been
changed to pass through "all" and all the data in the database has been
updated, we can update the code to stop supporting both values.
