Redis settings from the .env file are currently ignored

Currently only default values are used.
Places where it is used:
- `.env` and `.env_sample`
- `main.py`
- `celery_app.py`
- all `tasks.py` files

It's probably best to combine the various Redis URL components in the env file into one setting:
`REDIS_URL =  "redis://localhost:6379/0"`