Atlas: serve error page for internal login
    
    Grafana now uses OIDC authentication. Avoid directing users to
    idm.i.o.o (which cannot be reached from the internet) and serve
    an informative error page instead.
    The new page and backend is kept generic to allow for easy re-use with
    other internal services which might find public linking in the future.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>