move (non-api) clients (inc redis) from app/__init__.py to extensions

when clients are defined in app/__init__.py, it increases the chance of 
cyclical imports. By moving module level client singletons out to a 
separate extensions file, we stop cyclical imports, but keep the same 
code flow - the clients are still initialised in `create_app` in 
`__init__.py`.

The redis client in particular is no longer separate - previously redis 
was set up on the `NotifyAdminAPIClient` base class, but now there's one 
singleton in `app.extensions`. This was done so that we can access redis 
from outside of the existing clients.

app/extensions.py

@@ -0,0 +1,11 @@
+from notifications_utils.clients.antivirus.antivirus_client import (
+    AntivirusClient,
+)
+from notifications_utils.clients.redis.redis_client import RedisClient
+from notifications_utils.clients.statsd.statsd_client import StatsdClient
+from notifications_utils.clients.zendesk.zendesk_client import ZendeskClient
+
+antivirus_client = AntivirusClient()
+statsd_client = StatsdClient()
+zendesk_client = ZendeskClient()
+redis_client = RedisClient()