Stageless pipelines visualization improvements

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Currently a "stageless pipeline" (pipeline defined only using the needs keyword, no stages) is treated as if it had a single "test" stage with all jobs inside. Example:

As a result, the pipelines mini-graphs on the pipelines page or in merge requests or commits view only show a single node:

However the individual pipeline page shows nicely the jobs DAG using the job dependencies view:

Proposal

Treat stageless pipelines as if they had "nameless stages", like they are rendered in the job dependency view, to improve the visualization of the pipelines graphs on the pipelines page and in merge requests. In addition the "related jobs" section of a job's page could show only the jobs in the same nameless stage.

I believe this is relevant for &15737, FYI @sunjungp

Closely related: #359936 but with smaller scope (re-name default stage)

Thank you!