There was a bug where you would type "is:private je" into the search
suggestion and see undefined:jesstess@zulip.com. Now we use
the "pm-with" operator. The search suggestions for people are kind
of complicated now, because there is some overlap between
get_private_suggestions and get_person_suggestions.
(imported from commit 7d330f34f4a433995420de6eb90cb41229b70272)