Make the login page do something useful without using the authn client

[joachimvh]

The login page only works if you're in an authn session (started by session.login) so the OIDC library already has the relevant metadata. Potentially we can also set the correct cookie outside of that but would have to investigate if this is possible and valid OIDC.

[joachimvh]

As a reminder for when looking into this: the OIDC library provides a function provider.setProviderSession to generate relevant cookies outside of an auth session. Potentially we can do something with this.

See https://github.com/panva/node-oidc-provider/blob/v6.x/docs/README.md#user-flows

[joachimvh]

I'm thinking the most useful thing we could do would be to just throw a clearer error in case there is no session. Besides that most issues surrounding this will probably disappear once #880 is resolved to hide the login page on the registration page if it is not relevant then. After that it should be impossible for a normal user to still find that login page (only way to find it would be to be redirected during auth procedure).

We probably do want a default login page at some point to have pod controls, but I'm thinking this should probably be a different login page since we need to handle cookies differently there.