1.3.0 breaks `redis:///` Unix socket URLs in CACHE_URL

Steps to reproduce

1. Update 1.2.x to 1.3.0.
2. Attempt to login.

What happens?

The login stalls and eventually fails.

What is expected?

The login should proceed normally.

Context

Funkwhale version(s) affected: 1.3.0

RuntimeError: Unable to create a new session key. It is likely that the cache is unavailable.

Forcing the cache to expose errors yields:

redis.exceptions.ConnectionError: Error 111 connecting to localhost:6379. Connection refused.

Funkwhale 1.2.x used django-environ to sanitize redis:///run/redis/redis.sock?db=0 to unix:///run/redis/redis.sock?db=0.

This commit removed that behavior in order to allow CELERY_BROKER_URL to default to CACHE_URL for convenience:

30286720

That's perfectly fine but the sample config still refers to the redis:/// style URL as if it will work, leading to confusion.