[backend] Refactoring of the local scheduler mechanism

beat/web/backend/admin.py

beat/web/backend/api.py

beat/web/backend/api_urls.py

beat/web/backend/helpers.py

beat/web/backend/local_scheduler.py

beat/web/backend/migrations/0006_localschedulerprocesses.py

beat/web/backend/models/__init__.py

beat/web/backend/models/local_scheduler.py

beat/web/backend/templates/backend/scheduler.html

beat/web/backend/templatetags/backend_tags.py

beat/web/backend/tests/test_helpers.py

beat/web/backend/tests/test_scheduler.py

beat/web/backend/views.py

beat/web/experiments/models/experiment.py

beat/web/experiments/models/result.py

beat/web/experiments/serializers.py

beat/web/experiments/static/experiments/js/panels.js

beat/web/scripts/scheduler.py

beat/web/settings/settings.py