Add the loading skeleton for dashboards to components
### Problem
Loading skeleton for dashboards was added as a _region component_ within `/regions` in https://gitlab.com/gitlab-org/gitlab-ui/merge_requests/191. 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.
issue