Replace dashboard layout component with GitLab UI version

Problem to solve

With Add dashboard grid to GitLab UI (#542162 - closed), we added the dashboard layout component to GitLab UI. We now need to replace the usages of the old dashboard layout component with the new one.

Implementation plan

frontend - 2️⃣

1. Remove old dashboard layout component from vue_shared.
2. Replace uses of the old dashboard layout component with the one from GitLab UI.
3. Update specs.
4. Update docs.