[backend] Refactoring of the Job mechanism

beat/web/backend/admin.py

beat/web/backend/helpers.py

beat/web/backend/migrations/0005_job_modifications.py

beat/web/backend/models/job.py

beat/web/backend/models/queue.py

beat/web/backend/models/worker.py

beat/web/backend/schedule.py

beat/web/backend/state.py

beat/web/backend/templatetags/backend_tags.py

beat/web/backend/tests.py

beat/web/backend/tests/__init__.py

beat/web/backend/tests/common.py

beat/web/backend/tests/test_api.py

beat/web/backend/tests/test_cache.py

beat/web/backend/tests/test_helpers.py

beat/web/backend/tests/test_scheduler.py

beat/web/backend/tests/test_setup.py

beat/web/backend/utils.py

beat/web/experiments/models/block.py

beat/web/experiments/models/experiment.py