Since letter jobs from the API aren't created by any single individual,
lets make created_by nullable. Note: We'll have to make sure that we
update the admin app to handle these jobs nicely
when functions get as big as that, it's confusing to try and work out what
things are what. By including a * as the first arg, we require that anyone
calling the function has to use kwargs to reference the parameters
similar to sms/email, however, for consistency with response values
and internal storage, rather than supplying an "email_address" field
or a "phone_number" field, supply an "address_line_1" and "postcode"
field within the personalisation object.
