Implement Alert according to Pajamas

• Epic: Pajamas component: Alerts - Build
• Component's specifications: https://design.gitlab.com/components/alert/
• Sketch measurements: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/alerts-spec-previews/

Preview

Key points

• The dismiss button (x) is optional
• At a later stage, we might add additional actions next to the x, ie: gitlab-design#626 (closed) (no need to worry about this for now but let's keep it in mind while building the component)
• The alert should always contain an icon representing its type
  • Error: https://gitlab-org.gitlab.io/gitlab-svgs/?q=~error
  • Warning: https://gitlab-org.gitlab.io/gitlab-svgs/?q=~warning
  • Information: https://gitlab-org.gitlab.io/gitlab-svgs/?q=~information-o
  • Success: https://gitlab-org.gitlab.io/gitlab-svgs/?q=~status_success
  • Tip: https://gitlab-org.gitlab.io/gitlab-svgs/?q=~bulb
• Alert icon: Always center-aligned (vertically) with the first line in the alert, paragraph or title.

/cc @andyvolpe