Provide (in descending priority order):
1. an StoryTitleAttributedString object which provides attributed string containing the (optional) image and a link, for drops into apps such as Notes, or if rich text is unsupported, Story Title <story URL>.
2. a URL (alone)
This avoids the issue where a “2” or “3” is displayed and multiple items appear to be being dragged (see #1072).
Correctly return an empty array rather than nil if we have nothing to drag.
