Initialise clients outside the app

This avoids the annoying problem where you can’t import a client unless
the app has already been initialised.

app/notify_client/email_branding_client.py

@@ -50,3 +50,6 @@ class EmailBrandingClient(NotifyAdminAPIClient):
             "brand_type": brand_type
         }
         return self.post(url="/email-branding/{}".format(branding_id), data=data)
+
+
+email_branding_client = EmailBrandingClient()