Resurrect setup check for cluster.mailers

Summary:
D19940 removed this file entirely, which has led to at least one user who was unsure how to proceed now that `cluster.mailers` is required for outbound mail: https://discourse.phabricator-community.org/t/invalid-argument-supplied-for-foreach-phabricatormetamtamail-php/2287

This isn't //always// a setup issue for installs that don't care about sending mail, but this at least this gives a sporting chance to users who don't follow the changelogs.

Also, I'm not sure if there's a way to use `pht()` to generate links; right now the phurl is just in plain text.

Test Plan: Removed `cluster.mailers` config; observed expected setup issue.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19964

src/__phutil_library_map__.php

@@ -3411,6 +3411,7 @@ phutil_register_library_map(array(
     'PhabricatorMailReceiverTestCase' => 'applications/metamta/receiver/__tests__/PhabricatorMailReceiverTestCase.php',
     'PhabricatorMailReplyHandler' => 'applications/metamta/replyhandler/PhabricatorMailReplyHandler.php',
     'PhabricatorMailRoutingRule' => 'applications/metamta/constants/PhabricatorMailRoutingRule.php',
+    'PhabricatorMailSetupCheck' => 'applications/config/check/PhabricatorMailSetupCheck.php',
     'PhabricatorMailStamp' => 'applications/metamta/stamp/PhabricatorMailStamp.php',
     'PhabricatorMailTarget' => 'applications/metamta/replyhandler/PhabricatorMailTarget.php',
     'PhabricatorMailUtil' => 'applications/metamta/util/PhabricatorMailUtil.php',
@@ -9223,6 +9224,7 @@ phutil_register_library_map(array(
     'PhabricatorMailReceiverTestCase' => 'PhabricatorTestCase',
     'PhabricatorMailReplyHandler' => 'Phobject',
     'PhabricatorMailRoutingRule' => 'Phobject',
+    'PhabricatorMailSetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorMailStamp' => 'Phobject',
     'PhabricatorMailTarget' => 'Phobject',
     'PhabricatorMailUtil' => 'Phobject',

src/applications/config/check/PhabricatorMailSetupCheck.php

@@ -0,0 +1,24 @@
+<?php
+
+final class PhabricatorMailSetupCheck extends PhabricatorSetupCheck {
+
+  public function getDefaultGroup() {
+    return self::GROUP_OTHER;
+  }
+
+  protected function executeChecks() {
+    if (PhabricatorEnv::getEnvConfig('cluster.mailers')) {
+      return;
+    }
+
+    $message = pht(
+      'You haven\'t configured mailers yet, so Phabricator won\'t be able '.
+      'to send outbound mail or receive inbound mail. See the '.
+      'configuration setting cluster.mailers for details.');
+
+    $this->newIssue('cluster.mailers')
+      ->setName(pht('Mailers Not Configured'))
+      ->setMessage($message)
+      ->addPhabricatorConfig('cluster.mailers');
+  }
+}