Management script to create a dummy worker/worker version

Part of https://gitlab.com/arkindex/tasks/-/issues/76

Build a management script: ./manage.py fake_worker_version --url=https://fake/xxx --name="Demo worker XX" that produces the full chain of model dependency:

• a repo with the specified url
• a worker with the specified name
• a revision with a random hash
• a worker version with a random ID

The worker version built should be displayed