Rename hunt_id_index to is_hunt_index

Backed, of course, by a new is_hunt attribute.

The rationale here is that the whole point of this index is to be a
sparse attribute so that the code can efficiently query all existing
hunts. But meanwhile, it's useful for puzzles to have a "hunt_id"
attribute, which would confict with the goal of having "hunt_id" be
sparse and populated only for hunts and not puzzles.

So the fix here is to make hunt_id universal, and instead have a
separate "is_hunt" attribute which is the sparse one, (and will
clearly never be populated for a puzzle).