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.