Add emergency banner component

[simonwhatley]

Example:

Code can be found here:

• https://github.com/alphagov/static/blob/master/app/assets/stylesheets/components/_emergency-banner.scss
• https://github.com/alphagov/static/blob/master/app/views/components/_emergency_banner.html.erb

More info on GOV.UK Static:

• https://docs.publishing.service.gov.uk/apps/static.html