Give data exporters a header row

Summary:
Depends on D18951. Ref T13049. When we export to CSV or plain text, add a header row in the first line of the file to explain what each column means. This often isn't obvious with PHIDs, etc.

JSON has keys and is essentially self-labeling, so don't do anything special.

Test Plan: Exported CSV and text, saw new headers. Exported JSON, no changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18952

src/applications/search/controller/PhabricatorApplicationSearchController.php

@@ -454,6 +454,7 @@ final class PhabricatorApplicationSearchController
         $field_list = $engine->newExportFieldList();
         $field_list = mpull($field_list, null, 'getKey');
 
+        $format->addHeaders($field_list);
         for ($ii = 0; $ii < count($objects); $ii++) {
           $format->addObject($objects[$ii], $field_list, $export_data[$ii]);
         }