Support `object` and `link` values for `Question.closed`

[warriordog]

Apparently the presence of those values is meant to imply "true". We need to expand FlagWithTimestamp to support this case.

• as:closed is wrongly defined in the normative context as only being xsd:datetime w3c/activitystreams#542
• https://cosocial.ca/@evan/111656019397878971