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

[VonnyJap]

What happened:

There is no practical method to stop all all active builds within a stage other than manually stopping each running build within that stage.

What you expected to happen:

Give an option to at the top right of stage box to allow the option to stop a stage which will lead to halting all running builds.

How to reproduce it:

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