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/notify_client/job_api_client.py

@@ -1,5 +1,6 @@
 from collections import defaultdict
 
+from app.extensions import redis_client
 from app.notify_client import NotifyAdminAPIClient, _attach_current_user, cache
 
 
@@ -96,8 +97,7 @@ class JobApiClient(NotifyAdminAPIClient):
         return bool(self.get_jobs(service_id)['data'])
 
     def create_job(self, job_id, service_id, scheduled_for=None):
-
-        self.redis_client.set(
+        redis_client.set(
             'has_jobs-{}'.format(service_id),
             b'true',
             ex=cache.TTL,