Where a field is a BooleanField or a NullBooleanField inference should
generate a lookup list with `[Yes, No]` and `[Yes, No, Unknown]` as
options (for radio buttons).

This will require changes to templates/radio.html so that Unkown does
not use Unknown as a value, instead leaving the value empty so that we
know the intended value is None.

This would be easier if lookuplists were tuples (like choices in Django)
so I'm considering implementing support for lists of tuples (as well as
the current list of strings) for lookuplists - the alternative is too
much logic in the template and special-casing the string 'Unknown'

Will fix #1540

refs #1540