Extension idea: Generating MDN via JMAP

[brong]

Request from Linagora: way to generate MDN via JMAP. They have added a custom extension to earlier JMAP version.

[cjnewman]

It's pretty easy to generate an MDN in either the client or server, but the desirable atomic behavior with respect to the $MDNSent keyword is much easier to implement server side. For that reason, I think there's a solid case to formally specify this and do this in JMAP. Since JMAP mail has MDN tracking, MDN generation increases consistency.