Add GraphQL field for sidebar counts

What does this MR do and why?

Add GraphQL field for sidebar counts

This allows us to render the sidebar counts asynchronously so we don't block the rest of the page.

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #498901

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Example query:

{
  namespace(fullPath: "flightjs/test-project") {
    sidebar {
      openIssuesCount
      openMergeRequestsCount
      openEpicsCount
    }
  }
}

You can pass this a group / project path and verify that the counts are the same as the one you see in the sidebar.