Support EU domains for Mailgun API

Summary:
See <https://discourse.phabricator-community.org/t/mailgun-eu-zone-not-working/2332/4>.

Mailgun has a couple of API domains depending on where your account is based.

Test Plan:
  - Sent normal mail successfully with my non-EU account.
  - Waiting on user confirmation that this works in the EU.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20055

src/applications/metamta/adapter/PhabricatorMailMailgunAdapter.php

@@ -24,6 +24,7 @@ final class PhabricatorMailMailgunAdapter
       array(
         'api-key' => 'string',
         'domain' => 'string',
+        'api-hostname' => 'string',
       ));
   }
 
@@ -31,12 +32,14 @@ final class PhabricatorMailMailgunAdapter
     return array(
       'api-key' => null,
       'domain' => null,
+      'api-hostname' => 'api.mailgun.net',
     );
   }
 
   public function sendMessage(PhabricatorMailExternalMessage $message) {
     $api_key = $this->getOption('api-key');
     $domain = $this->getOption('domain');
+    $api_hostname = $this->getOption('api-hostname');
     $params = array();
 
     $subject = $message->getSubject();
@@ -92,7 +95,8 @@ final class PhabricatorMailMailgunAdapter
     }
 
     $mailgun_uri = urisprintf(
-      'https://api.mailgun.net/v2/%s/messages',
+      'https://%s/v2/%s/messages',
+      $api_hostname,
       $domain);
 
     $future = id(new HTTPSFuture($mailgun_uri, $params))

src/docs/user/configuration/configuring_outbound_email.diviner

@@ -227,6 +227,9 @@ To use this mailer, set `type` to `mailgun`, then configure these `options`:
 
   - `api-key`: Required string. Your Mailgun API key.
   - `domain`: Required string. Your Mailgun domain.
+  - `api-hostname`: Optional string. Defaults to "api.mailgun.net". If your
+    account is in another region (like the EU), you may need to specify a
+    different hostname. Consult the Mailgun documentation.
 
 
 Mailer: Amazon SES