Currently, a moderator can send a one-way message to a user, but the user cannot respond on-site. Sometimes mods really want the user to be able to respond, however. Currently they have to provide some out-of-band contact info, like an email address. That's not ideal, either for the mod (who has to expose personal info) or the network (important info about a user interaction is not available to other mods/admins). We would much rather have the user be able to respond on-site, and for all mods to be able to see the exchange.

This change request was initially bigger; I've split part of it off into #757.