calamari_web: improve static file serving

* Remove the login_required on serving static files, the UI
  handles the case where it's loaded without authentication.
* Less repetition in setting up the URLs, just have a list of paths
  to serve (but still with the special case for dashboard :-/)
* Give a helpful message when UI files are not found, to reduce
  the "why is it saying /login/ not found?" queries.

Signed-off-by: John Spray <john.spray@redhat.com>