Add .cost() to notification model

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.

tests/app/v2/notifications/test_notification_schemas.py

@@ -3,7 +3,6 @@ import uuid
 import pytest
 from flask import json
 from jsonschema import ValidationError
-from notifications_utils.recipients import InvalidPhoneError, InvalidEmailError
 
 from app.v2.notifications.notification_schemas import post_sms_request, post_sms_response, post_email_request, \
     post_email_response