Make text black in error/danger banners
Make text black in error/danger banners

For consistency with what GOV.UK Elements does, the text in our banners
should be black, not red. See examples here:
http://govuk-elements.herokuapp.com/errors/#summarise-errors

This also makes us consistent with what Pay are doing.

alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin