Allow scheduled status jobs to be cancelled from the UI

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Problem

When a job is in the scheduled status we can't cancel it via the UI but it is cancelable via the backend processes because we don't actually enforce this in the CancelPipelineService or at the processable state machine level.

See: #477972 (closed)

Proposal

• Add :scheduled to CANCELABLE_STATUSES
• Adjust build state machines so that we can transition manaul to canceled to canceling via finish_cancel and start_cancel

Possible follow-up:

Only cancel cancelable jobs when canceling a pipeline. This will prevent us from attempting to run cancel on jobs with a completed status which leads to an error when it hits the state machine.