Show ready to merge badge on merge request dashboard

What does this MR do and why?

Updates the merge request status badge on the dashboard to show a Ready to merge badge when all merge checks have passed.

Note this is behind the merge_request_dashboard_merge_checks feature flag to make sure the query does not timeout.

References

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• #518710 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

• Enable merge_request_dashboard feature flag
• Enable merge_request_dashboard_merge_checks feature flag
• Visit https://gdk.test:3000/dashboard/merge_requests
• If a merge request is ready to merge a badge should be shown