Implement Broadcast Message according to Pajamas

• Related issue: Create, Build, and Style > Broadcast messages
• Epics:
  • Pajamas Component: Broadcast Messages - Build
  • Pajamas Component: Broadcast Messages - Style
• Component's specifications: https://design.gitlab.com/components/broadcast-messages
• Sketch Measure Previews: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/broadcast-messages-spec-previews/
• Design Pattern Library: Broadcast messages: gitlab-design#156 (closed)

Preview

When there is only one broadcast message	When there are 1+ broadcast messages

Key points

1. TBD