Add social-auth-core[saml] as a dependency

[noisecapella]

Unless this package is included in projects using ol-django's authentication app, there are some missing packages like onelogin for example. We should include social-auth-core[saml] as an explicit dependency since the authentication code won't work without it. What do you think @rhysyngsun?

[rhysyngsun]

@noisecapella I think it should probably be an optional dependency, but the auth app should have an opinion about which versions it supports