Assign existing worker activity to a newly created process

Refs https://gitlab.com/teklia/arkindex/backend/-/issues/1350
Refs https://gitlab.com/teklia/requests/-/issues/766

Instead of stealing the worker activities during UpdateWorkerActivoty, we should implement the re-assignation of existing WorkerActivity to a new process directly in the initialize_worker_activity task.

This will probably require raw sql to use insert or update (but please try the django implem 🤞 )

The UpdateWorkerActivity endpoint should also be updated to remove the stealing part.