Rename primary-key attribute from PK to hunt_id

Rename primary-key attribute from PK to hunt_id

I was just about to add a separate attribut to every item in the
database named hunt_id when I realized that that was totally redundant
with our existing PK attribute (whose values were simply the hunt_id
prefixed by "hunt-" in all cases).

Since we're not using the PK field in any generalized sense, (that is,
it's _always_ populated with the hunt_id value), it's much simpler and
more efficient to just name it "hunt_id" and store the hunt_id valeu
ther directly (without any prefix).