Delete parent/child pipelines

Follow up issue from !21365 (comment 257876879)

• We should allow only the parent pipeline to be deleted
• When deleting the parent pipeline we should also delete the child pipelines
• A child pipeline cannot be deleted

Ensure we prevent that a child pipeline is deleted by making the necessary changes to the pipeline policy so we can stop it via API and controller. Also in the UI we need to disable the Delete pipeline button with a tooltip if the pipeline cannot be deleted.

Proposal

As we did for Cascade parent/child pipeline cancellation (#273378 - closed) we can iterate through all child pipelines (including nested ones) and kick off an async deletion for each child pipeline.