Add a dropdown menu from UI to allow a stage to be disabled

[VonnyJap]

What happened:

There is no practical method to deactivate all jobs within a stage other than individually addressing each job within that stage.

What you expected to happen:

Give an option to at the top right of stage box to allow the option to deactivate a stage, similar to how we allow for job nodes.

How to reproduce it:

See pipeline example https://cd.screwdriver.cd/pipelines/14176/events/802455

[github-actions[bot]]

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