Display flash messages at the top if there are any.

@ourmaninamsterdam we are using the same markup and style as the opt-out bar at the moment but we need to update this.
[WIP] Add HTML for global alert
Remove layout containers to allow the alert to fit full width
Add Global alert heading flag, modifier class and <p> formatting
Add global-alert to style guide
Merge pull request #460 from moneyadviceservice/feature/global-alerts

Feature/global alerts