R1:ab8f7907dee8

Herald - add support for application emails.

Summary:
Fixes T5039. The trick / possibly lame part here is we only match 1 application email and its undefined which one. e.g. if a user emails us at address x, y, and z only one of those will pick up the mail. Ergo, don't let users define non-sensical herald conditions like "matches all". Also document what I think was non-intuitive about the code with an inline comment; we have to return an array with just a phid from an object and out of context it feels…