Add mailKey to macros

Summary:
If you have private replies on and a Macro reply handler set, we try to access `getMailKey()` and fail. See P1039 for a trace.

(Thanks to @Korvin for picking this up.)

Test Plan: Set configuration, repro'd the exception, applied the patch, then disabled/enabled a macro.

Reviewers: btrahan

Reviewed By: btrahan

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D7896