Revert "fixes: hidden pages are shown in navigation"

This reverts commit 1d82c50500ed39fb0497a9bf8285ce8166870cee.

It turned out that the invisible pages were only shown because we had
an always present current_alchemy_user with admin rights. Since we
changed this, the alchemy way of showing/hiding pages using
accessible_by works again and provides better features (having a
preview as a logged in user).
Let rails do the error page rendering

After the last change in the render_404 (always rendering without
layout), this basically just did what ActionDispatch::PublicExceptions
(the default exceptions_app) does. Without this custom code, the app
has greater flexibility, eg by being allowed to set a custom exceptions_app.