Ensure clients have rate limit enforced

- rate limiting is a hard number per day
- not limited in terms of rate of request
- limit is a single number held against the service
- every notification counts against the limit, regardless of type
- return a 429 if limit exceeded.

app/__init__.py

@@ -14,6 +14,7 @@ from app.clients.email.aws_ses import AwsSesClient
 from app.encryption import Encryption
 
 DATETIME_FORMAT = "%Y-%m-%dT%H:%M:%S.%f"
+DATE_FORMAT = "%Y-%m-%d"
 
 db = SQLAlchemy()
 ma = Marshmallow()