Allow moderators to queue offline messages

[Sheeo]

Messages should pop up when the user comes back online

[LegionDarrath]

This should really only be a feature for moderators where it is a literal pop up saying a [Moderator name] messaged you while you were offline: [Message text] and then an OK button that prevents users from getting past that box unless they click OK.

Full offline messaging is not really needed plus if the user is forced to interact with the message in case of a warning they can't deny having seen the message.

Ideally there'll be a way for all mods to track any such messages being sent and their receipt status but honestly just having the ability to send messages like this would be appreciated by the mod team.