Change cache_url to redis_url

[filippo-20tab]

The current secret holds the CACHE_URL value, as expected by the django-cache-url package. This prevents other potential components (e.g. Celery) to access the Redis server URL, which is contained in the CACHE_URL value but can not easily be used as-is. Therefore I suggest storing a REDIS_URL value with the Redis server URL, and then populate the CACHE_URL environment variable from it in the Django deployment, as proposed in the twin PR in the Django-Talos subrepo.