Use consistent naming - Source not Stack

From a customer's perspective we've settled on the name "source" for a electric connection to a database, but we also call it "stack". It seems to be a frequent root of issues and requiring converting `stack_id` to `source_id`. It would seem simpler to just always call it "source". Also "source" probably makes more sense than "stack" since it's the part that's for a specific source, not the entire stack, so `SourceSupervisor` makes more sense than `StackSupervisor` etc.