feat(matrix_chat): remove references to django-cas-server
The [django-cas-server](https://github.com/nitmir/django-cas-server/) works nicely as a single-sign-on session provider, but since its session is not related to the grouprise session (in the same Django application), it requires an additional login activity (see [django-cas-server#70](https://github.com/nitmir/django-cas-server/issues/70)). Thus we stick with the (local-only) grouprise authentication module for matrix-synapse for now. In the future we may want to explore the possibility of using an OIDC provider module, which is tied to our grouprise session.
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