Add issued date to Medical Safety Alerts

Adds issued date to medical safety alerts as a required fields and
exports it to the relevant APIS (Rummager, Content API)

Trello ticket:
https://trello.com/c/QFE3nYlc/361-add-issued-date-to-medical-safety-alerts-2
Merge pull request #281 from alphagov/feature/add-issued-date-to-medical-safety-alerts

Add issued date to Medical Safety Alerts