Render styles for authenticated users only.

Let's discuss first. AFAIK these stylesheets are required for rendering the cropping view, so we could include them using a ``metal:fill-slot="style-slot"`` in template directly. 

But if users are using the ``prepOverlay`` stuff for cropping in an overlay these styles would not be included. So for now i think it's the safest way to just render them for authenticated users. 

Anonymous users will never crop images - right? ;-)