Allow "!accept" to be enabled through configuration

Summary: For reasons explained in the config I've omitted this from the default
action set, but it's trivial to support it. See D916.

Test Plan: Commented on a revision, was informed I could "!accept" in the email.
Used "!accept" to accept the revision.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 928

src/applications/differential/replyhandler/DifferentialReplyHandler.php

@@ -88,7 +88,7 @@ class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
   }
 
   public function getSupportedCommands() {
-    return array(
+    $actions = array(
       DifferentialAction::ACTION_COMMENT,
       DifferentialAction::ACTION_REJECT,
       DifferentialAction::ACTION_ABANDON,
@@ -97,6 +97,12 @@ class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
       DifferentialAction::ACTION_RETHINK,
       'unsubscribe',
     );
+
+    if (PhabricatorEnv::getEnvConfig('differential.enable-email-accept')) {
+      $actions[] = DifferentialAction::ACTION_ACCEPT;
+    }
+
+    return $actions;
   }
 
   public function receiveEmail(PhabricatorMetaMTAReceivedMail $mail) {