Fix worker permissions in ListWorkers and RetrieveWorker

Per the docs, a Guest role has no meaning on workers, so guest access should not be handled separately. This caused me trouble when implementing #1644 (closed), so I updated the existing ListWorkers, CreateWorker and RetrieveWorker to remove the WorkerACLMixin and use the newer permission checks we started to implement for models and farms.

Note that the WorkerVersion, WorkerConfiguration and template APIs are still using the mixin and may still have this issue, but that would be better handled in a separate issue to completely remove the WorkerACLMixin.