Add the loading skeleton for dashboards to components

Problem

Loading skeleton for dashboards was added as a region component within /regions in !191 (merged). However, the design system includes this as a single component on /components. One of them is wrong, or both.

See https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/regions-dashboard-skeleton--default v. https://design.gitlab.com/components/skeleton-loader.

Proposal

Maybe it's worth adding this both as a single component and a region component. Having a single loading skeleton card on small viewports (mobile) sounds good.

1. Add the skeleton for dashboards as a single component.
2. Keep the 3-card-version as a region component.