Add skeleton loader to Analytics -> CI/CD page

What does this MR do?

This adds a skeleton loader for the overall statistics section of the Project -> Analytics -> CI/CD page, this is behind the graphql_pipeline_analytics feature flag introduced in !48267 (merged)

Screenshots (strongly suggested)

Screen_Recording_2020-12-10_at_15.24.44

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content