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/main/views/feedback.py

@@ -4,12 +4,8 @@ import pytz
 from flask import abort, redirect, render_template, request, session, url_for
 from flask_login import current_user
 
-from app import (
-    convert_to_boolean,
-    current_service,
-    service_api_client,
-    zendesk_client,
-)
+from app import convert_to_boolean, current_service, service_api_client
+from app.extensions import zendesk_client
 from app.main import main
 from app.main.forms import Feedback, Problem, SupportType, Triage