Watch `django-saml2-auth` for a new release and update tator dependencies

[henxing]

Currently, django-saml2-auth is provided by a CVision-maintained fork of https://github.com/grafana/django-saml2-auth. PR#72 in that repo contains the changes from the CVision fork, so once a new release of it shows up on PyPI, remove the custom build in the Dockerfile and replace it with the correct pip install.