Escalated flags: add a way to respond to the moderator

[cellio]

A moderator can escalate a flag to admins and add a note explaining why it's being escalated. In the escalations I've seen, the moderator often asks a question. As an admin, I would like to be able to respond to the moderator as part of the flag interaction, instead of by looking for a private way to contact the mod. In addition to being easier, this would make the information available to other mods too.

The modtools feature that @luap42 is currently working on (#767) will add private comment threads (detached from posts) to support mod messages. Perhaps, after that lands, there would be some way to hook flag escalations into it.