Fix an issue where the "viewer" is not passed to Bulk Edit controls properly

Summary:
See PHI1442. If you have a bulk-editable datasource field with a composite datasource, it can currently fatal on the bulk edit workflow because the viewer is not passed correctly.

The error looks something like this:

> Argument 1 passed to PhabricatorDatasourceEngine::setViewer() must be an instance of PhabricatorUser, null given, called in /Users/epriestley/dev/core/lib/phabricator/src/applications/typeahead/datasource/PhabricatorTypeaheadCompositeDatasource.php on line 231

Test Plan: Configured a Maniphest custom field with a composite datasource, then tried a bulk edit. Things worked cleanly instead of fataling.

Differential Revision: https://secure.phabricator.com/D20841

src/applications/transactions/bulk/type/BulkTokenizerParameterType.php

@@ -22,7 +22,8 @@ final class BulkTokenizerParameterType
     $template = new AphrontTokenizerTemplateView();
     $template_markup = $template->render();
 
-    $datasource = $this->getDatasource();
+    $datasource = $this->getDatasource()
+      ->setViewer($this->getViewer());
 
     return array(
       'markup' => (string)hsprintf('%s', $template_markup),

src/applications/transactions/editengine/PhabricatorEditEngine.php

@@ -2523,6 +2523,8 @@ abstract class PhabricatorEditEngine
   }
 
   final public function newBulkEditMap() {
+    $viewer = $this->getViewer();
+
     $config = $this->loadDefaultConfiguration();
     if (!$config) {
       throw new Exception(
@@ -2542,6 +2544,8 @@ abstract class PhabricatorEditEngine
         continue;
       }
 
+      $bulk_type->setViewer($viewer);
+
       $bulk_label = $type->getBulkEditLabel();
       if ($bulk_label === null) {
         continue;