Pswadi/add stop n approve

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/cc @zendesk/samson

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@jwswj - yes, given that can_be_stopped_by?() is called elsewhere, what about a sister function that is called only in the context of Approve/Stop, so that a person can either approve or stop when that person is > viewer (e.g. a new function, can_be_approved_cancelled_by?()).

I agree that for already running jobs, or for jobs that are in a pending state due to a restart, we should remain with the current requirement of > deployer rights for stopping deployments.