Add API Broadcast endpoints

A new API endpoint for broadcast is required. The current backend handles it like this.

DashboardControl.php#L71-74

contentGateway->getDetail(ContentId::BROADCAST_MESSAGE);

Maybe with status 200 it provides the text and with status 204 to send it but inform the frontend it is not updated.

Required data and current use

Broadcast

• title—string
• body—string
• last_mod—timestamp