Don't send email to bots subscribed to diffs

Summary:
Occasionally a bot will get subscribed to a differential revision; when
this happens we shouldn't send them an email, because otherwise the
author of the diff will get a bounce email if the bot has an invalid
emal address.

Test Plan:
re-sent a message that had a bot on the cc list, saw that it was no longer
included when it got sent.

Reviewers: epriestley, jungejason, vrana

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2479

src/applications/phid/handle/data/PhabricatorObjectHandleData.php

@@ -171,7 +171,8 @@ final class PhabricatorObjectHandleData {
                 $user->getUsername().' ('.$user->getRealName().')');
               $handle->setAlternateID($user->getID());
               $handle->setComplete(true);
-              $handle->setDisabled($user->getIsDisabled());
+              $handle->setDisabled($user->getIsDisabled() ||
+                                   $user->getIsSystemAgent());
 
               $img_uri = idx($images, $user->getProfileImagePHID());
               if ($img_uri) {