Send Differential e-mails in user's language

Summary:
Works this way:

- Select users' language with multiplexing.
- Select default language otherwise (it can be different from current user's language).
- Build body and subject for each user individually.
- Set the original language after sending the mails.

Test Plan:
- Comment on a diff of user with custom translation.
- Set default to a custom translation. Comment on a diff of user with default translation.
- Set default to a default translation. Comment on a diff of user with default translation.

Repeat with/without multiplexing.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1139

Differential Revision: https://secure.phabricator.com/D2774

src/applications/base/controller/PhabricatorController.php

@@ -62,9 +62,7 @@ abstract class PhabricatorController extends AphrontController {
 
     $translation = $user->getTranslation();
     if ($translation &&
-        $translation != PhabricatorEnv::getEnvConfig('translation.provider') &&
-        class_exists($translation) &&
-        is_subclass_of($translation, 'PhabricatorTranslation')) {
+        $translation != PhabricatorEnv::getEnvConfig('translation.provider')) {
       $translation = newv($translation, array());
       PhutilTranslator::getInstance()
         ->setLanguage($translation->getLanguage())