public.json: Don't require drop.id, but do require .exclusivity and .geo

It looks like unprivileged users should see:

* Everything about open drops.
* The name, exclusivity, and location of semi-open drops, but not the
  drop's ID.
* Nothing about closed drops.
public.json: Add member count as drop.members

From this morning on IRC (time in PST):

07:39 < shofetim> re member counts, Christina felt that it was
  important to tell the customers how big the drop was, that people's
  comfort levels with a drop would be different depending on whether
  it was a gathering of a few friends, or something that more
  resembled a block party.  World readable permissions.