We use the same strategy Zulip already uses for starred messages,
namely, creating a new UserMessage row with the "historical" flag set
(which basically means Zulip can ignore this row for most purposes
that use UserMessage rows). The historical flag is ignored, however,
in determining which users' browsers to notify about new reactions,
and thus the user will get to see the reaction appear when they click
a message (and any reactions other users later add, as well!).
There's still something of a race here, in that if some users react to
a message while the user is looking at the unsubscribed stream but
before the user reacts to that message, those reactions will not be
displayed to that user (so counts will be a bit lower, or something).
This race feels small enough to ignore for now.
Fixes#3345.
f3af0fe635