[Enhancement] Improve OpenID-Connect login button

[FBasels]

The OIDC login button could need some improvement and clean up.
Some things that come in mind are:

• ID tokens are extracted, but not used in any way
• ID tokens are not validated
• there is no working logout functionality
• validation of access token exists (via the userinfo endpoint) but no verification if the token is issued to this client

In general we could make use of the Single Sign-On functionality of Keycloak or/and make use of a JS library.
Best solution would be to just use our openidconnect-singin button, but maybe this would require some changes in the whole code.