Include status info as icons in alerts without relying on colour

The standard alerts/notifications use colour alone to hint about the state of the message:

We use bootstrap classes to indicate "success, info, warning and danger".

In MDL-64494 we talked about including icons in each alert with some accessible alt text to convey this status information.

We should add this icon to the layout of each alert - and review each usage of an alert to make sure the additional text is useful and does not just duplicate the text of the message.