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/__init__.py

@@ -2,7 +2,6 @@ from flask_login import current_user
 from flask import has_request_context, request, abort
 from notifications_python_client.base import BaseAPIClient
 from notifications_python_client import __version__
-from notifications_utils.clients.redis.redis_client import RedisClient
 
 
 def _attach_current_user(data):
@@ -14,8 +13,6 @@ def _attach_current_user(data):
 
 class NotifyAdminAPIClient(BaseAPIClient):
 
-    redis_client = RedisClient()
-
     def __init__(self):
         super().__init__("a" * 73, "b")
 
@@ -24,8 +21,6 @@ class NotifyAdminAPIClient(BaseAPIClient):
         self.service_id = app.config['ADMIN_CLIENT_USER_NAME']
         self.api_key = app.config['ADMIN_CLIENT_SECRET']
         self.route_secret = app.config['ROUTE_SECRET_KEY_1']
-        self.redis_client.init_app(app)
-        self.redis_client.redis_store.decode_responses = True
 
     def generate_headers(self, api_token):
         headers = {