Stateful notifications

[julianbrost]

Probably not the best title, so let me try to explain what I mean by that.

It would be nice if notifications for the same incident could be grouped together, so that for example, with e-mail, follow-up notifications show up as a response to the previous e-mail and for a chat, messages for the same incident are within one thread.

In order to achieve this, it's probably necessary to provide some mechanism to notification plugins to keep some state, like for example the Message-ID of the last and/or first e-mail for an incident or some thread reference for other messengers.

The goal of this issue is to evaluate what exactly would be needed to achieve this and suggest how this could be implemented.