Fix ProcessWorkersActivity permissions and change URLs

While implementing #892 (closed), I noticed that while the docstring of ProcessWorkersActivity says it requires a guest access to a process, the code requires an admin access. I also struggled with the URLs, because the two worker activity endpoints have been added as if they are the activity lists and not some stats. The URLs have also been updated in the frontend on frontend!1104 (merged).

Additionally, both CorpusWorkersActivity and ProcessWorkersActivity should have their operation IDs changed to stay consistent with other endpoints and avoid a clash with the new list endpoint: they could be renamed ListCorpusWorkerStatistics or something similar. The serializer is already named WorkerStatistics…