Pause Button for Pipelines

Following up https://github.com/buildkite/feedback/issues/201

Having a way to quickly pause all automated builds of a pipeline would be very useful. For example, if there is a bug discovered with the pipeline or code related to it, I need to quickly lock down every possible way it can be triggered. This means touching other pipelines that trigger it, GitHub webhooks, scheduled builds, etc., which can be time consuming and error prone.