Resource DiscussionsDraftProperties is used to encapsulate properties which do not apply
directly to DiscussionsDrafts, but apply to the discussions objects they
are intended to become. For example, a DiscussionsDraft cannot have an
expiration date, but an Announcement can; if the DiscussionsDraft object is
intended to become a new Announcement, it can have an expiration date in its
properties that only takes effect when the draft is posted. Only the subtypes of
DiscussionsDraftProperties are used directly; these are TopicDraftProperties,
AnnouncementDraftProperties, and DiscussionsMessageDraftProperties.