Add a multi-select round field to puzzle creation

Actually, there are two round fields. The first is a free-form text
field for a comma-separated list of rounds. Each of these rounds will
be added to the database as an item with only two attributes:

	hunt_id: <HUNT_ID>
        SK: round-<ROUND>

The other field is a multi-select field populated with the names of
all rounds that already exist in the hunt. This makes for easy
selection without having to worry about typing in names identical with
what others typed previously.