Initialize Redis in searx/webapp.py

settings.yml:
* The default URL was unix:///usr/local/searxng-redis/run/redis.sock?db=0
* The default URL is now "false"

The default URL makes the log difficult to deal with:
if the admin didn't install a Redis instance, the logs record a false error.

It worked before because SearXNG initialized the Redis connection when the limiter started.

In this commit, SearXNG initializes Redis in searx/webapp.py
so various components can use Redis without taking care of the initialization step.

searx/settings.yml

@@ -78,7 +78,7 @@ server:
 
 redis:
   # https://redis-py.readthedocs.io/en/stable/connections.html#redis.client.Redis.from_url
-  url: unix:///usr/local/searxng-redis/run/redis.sock?db=0
+  url: false
 
 ui:
   # Custom static path - leave it blank if you didn't change