Workers activity link to processes

Add a process_id FK to the WorkerActivity model

• Two processes can be concurrents on a same element/worker-version
  • e.g. The first process creates a queued activity but the second may PATCH it to a "running" state with its own ID
  • This requires a better logic in the PATCH endpoint (with update lock)
• Deleting a process does not remove its related activities, but the FK is set to null
  • This should be probably done in an async task (the UPDATE took 2min10 for 4.6M activities on my laptop)
• Activities stats on a corpus can be filtered with the process_id flag