Convert /puzzle puzzle creation to all-one-table database schema

Convert /puzzle puzzle creation to all-one-table database schema

So we put the hunt ID into the PK field and the puzzle ID into the SK field,
(allowing puzzle items to be distinguished from hunt items in the table).

While doing this, we also disable the feature of inviting all hunt
users to the new puzzle. This is both because that code hasn't been
ported to the new schema, and also because that feature is planned to
be eliminated anyway.