Add OIDC scope setting

[mathildepommier]

For the moment, in the login view (OIDCLoginView), the scope variable is set to the value [”openid“] (in the get method).
This variable could be parameterized by the user via a scope field in Django settings, and its value retrieved as follows: self.opsettings.get(“scope”).
However, this variable could be optional and have [”openid“] as its default value.