Enable logout from OIDC Provider

[jennifer-richards]

The logout button ends the session with the RPC tool, but does not end the OIDC Provider's session. This means that, e.g., attempting to log out and then log back in as a different user is not possible without first manually logging out of the OP.

The datatracker is capable of RP-initiated logout as described here:
https://django-oidc-provider.readthedocs.io/en/master/sections/sessionmanagement.html

This should be available at least as an option when logging out from the RPC tool.