Add issued date to medical safety alerts

Add the issued date attribute to the medical safety alert date metadata
in the model so that it can be shown in the frontend. This corresponds
to this commit in specialist publisher:
alphagov/specialist-publisher#281

The trello ticket can be found here:
https://trello.com/c/QFE3nYlc/361-add-issued-date-to-medical-safety-alerts-2