Add issued date to medical safety alerts

Add the issued date attribute to the medical safety alaert date metadata
in  the presenter so that it can be shown in the frontend. This
corresponds to this commit in specialist publisher:
https://github.com/alphagov/specialist-publisher/pull/281

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