Add metadata headers to emails

[tomhughes]

The old notification system used to add a bunch of X-Fedmsg- headers to emails with metadata about the notification that could be used to filter emails and it would be great if the new one could as well rather than having to try and parse the information from the subject line.

If anything it's actually more important with the new system given the limited server side filtering which means there are often things you want to drop at the receiver.