Add hunt_id to the puzzle table (instead of orig_channel_name)

Add hunt_id to the puzzle table (instead of orig_channel_name)

A recent commit (to announce /solved in the main hunt channel)
expected to find hunt_id in this table, but it wasn't there. So we add
it in this commit. And while adding it, we drop the orig_channel_name
field, (which embedded both hunt_id and channel_id into a single field
in the database so would have made renaming of the hunt_id rather
awkward).