Ensure dateCreated property exists at index creation time

Summary:
Since the default query will sort on this when no query string is
attached we want to make sure the property at least exists.

Otherwise Elasticsearch yells at you: "No mapping found for [dateCreated]
in order to sort on" when you try to search for documents that haven't
been indexed yet.

Test Plan:
Searched for Mocks and Initiatives (no such documents exist in my index)
and got the error. After patching and reinitializing the index, the error
during querying went away.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11056

src/applications/search/engine/PhabricatorSearchEngineElastic.php

@@ -275,8 +275,12 @@ final class PhabricatorSearchEngineElastic extends PhabricatorSearchEngine {
     $types = array_keys(
       PhabricatorSearchApplicationSearchEngine::getIndexableDocumentTypes());
     foreach ($types as $type) {
+      // Use the custom trigram analyzer for the corpus of text
       $data['mappings'][$type]['properties']['field']['properties']['corpus'] =
         array( 'type' => 'string', 'analyzer' => 'custom_trigrams' );
+
+      // Ensure we have dateCreated since the default query requires it
+      $data['mappings'][$type]['properties']['dateCreated']['type'] = 'string';
     }
 
     return $data;