[backend] Refactoring: Split the models into separate, more manageable files

beat/web/backend/helpers.py

beat/web/backend/models/__init__.py

beat/web/backend/models/environment.py

beat/web/backend/models/job.py

beat/web/backend/models/queue.py

beat/web/backend/models/result.py

beat/web/backend/models/slot.py

beat/web/backend/models/worker.py

beat/web/backend/schedule.py

beat/web/backend/tests.py

beat/web/experiments/migrations/0009_block_status.py

beat/web/experiments/models/block.py

beat/web/experiments/models/experiment.py