allow setting permissions to restrict access to data but allow aggregate queries

[wawhal]

Thoughts @0x777 @rikinsk @coco98?

Can be achieved with views. But it would be very comfortable if we could set permissions such that one can make "only" aggregate queries. Makes the implementation of article_downvotes, article_upvotes etc joyful.

[rikinsk-zz]

I think this makes sense