refactor: Restructure backend as pure backend

backend/Dockerfile

backend/app/__init__.py

backend/app/app.py

backend/app/config.py

backend/app/extensions.py

backend/app/form2cookie.py

backend/app/lib/zip.py

backend/app/routes.py

backend/app/routes/__init__.py

backend/app/routes/cookiecutter.py

backend/app/settings.py

backend/app/templates/index.html

backend/autoapp.py

backend/cookiecutterform.py

backend/requirements.txt

backend/requirements/dev.txt

backend/requirements/prod.txt

backend/supervisord.conf

backend/supervisord_programs/gunicorn.conf