Create alert component that conforms to Pajamas design guidelines

gitlab-ui should include an "alert" component that conforms to Pajamas design guidelines:

Pajamas guidelines: https://design.gitlab.com/components/alerts/

Sketch mockups: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/alerts-spec-previews/

Here's an example stolen from the Sketch mockup above:

This alert has a very different visual style than the alerts provided by BootstrapVue: