Improved OIDC engine

We have improved OpenID Connect (OIDC), and rewritten the entire OIDC engine to make it even more reliable. Our OIDC flow now has better error tracing and gives more detailed feedback upon failures and configuration. Additionally, the changes improve our ability to extend the core functionality and add features specific to individual identity providers. While most of the changes in this release are not visible for users, administrators can now control which scopes to request in the user interface.