Send custom headers for author, reviewer and cc in Differential e-mails

Summary:
I want to flag messages which require an immediate action from me in e-mail
client.
It is currently not possible because Author and Reviewers fields are both in
To:.
So the filtering rule cannot recognize if I am the person who should take the
action.

This diff adds these headers:

- X-Differential-Author
- X-Differential-Reviewers
- X-Differential-CCs

Test Plan:
Send comment to the diff.
Verify X-Differential-* headers.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T808

Differential Revision: https://secure.phabricator.com/D1724

src/applications/differential/mail/base/DifferentialMail.php

@@ -100,6 +100,25 @@ abstract class DifferentialMail {
       $template->addHeader('X-Herald-Rules', $this->heraldRulesHeader);
     }
 
+    $revision = $this->revision;
+    if ($revision) {
+      if ($revision->getAuthorPHID()) {
+        $template->addHeader(
+          'X-Differential-Author',
+          '<'.$revision->getAuthorPHID().'>');
+      }
+      if ($revision->getReviewers()) {
+        $template->addHeader(
+          'X-Differential-Reviewers',
+          '<'.implode('>, <', $revision->getReviewers()).'>');
+      }
+      if ($revision->getCCPHIDs()) {
+        $template->addHeader(
+          'X-Differential-CCs',
+          '<'.implode('>, <', $revision->getCCPHIDs()).'>');
+      }
+    }
+
     $template->setIsBulk(true);
     $template->setRelatedPHID($this->getRevision()->getPHID());