Allow UI to reference a Workflow Snapshot instead of actual model when viewing runs

[stuartc]

When viewing a run that was executed against a snapshot, the UI should show that workflow and it's steps with the Job/Credential/Trigger/Edge that was used at the time of execution.

In addition, the WorkerChannel must use a WorkflowSnapshot as it's source of truth.

A proposed design would be to have the WorkflowSnapshot or similar module, act as an interface to retrieve information about it.

• [ ] #1843