We use django-ratelimit to limit per IP login attempts, and then we use
django-ratelimit-backend to limit per username login attempts. This
change replaces the usage of django-ratelimit-backend with another
instance of django-ratelimit so that both limits can be managed by one
library.

This is the first step in being able to fully excise
django-ratelimit-backend from edx-platform. Note that we're still using
the `RateLimitMixin` in openedx/core/djangoapps/oauth_dispatch/dot_overrides/backends.py
because studio and the admin UI still relies on that for rate limiting.
Those login paths will have to be updated before we can remove the mixin
from our auth backend.