Preview the action-items messages

Admins and owners: from the newest meeting you can open that has action items with a person, who would get a direct message with their own items after it (people: name, whether they're a member, how many items, and how Markdrop would find them in Slack: linked, email, name or none), and what the fullest message would read (message: its items with due dates, open_link for a member, organizer to ask for everyone else). Nothing is sent and Slack isn't asked. meeting is null when no meeting has such items yet.