Issue search support elasticsearch (#9428)

* Issue search support elasticsearch

* Fix lint

* Add indexer name on app.ini

* add a warnning on SearchIssuesByKeyword

* improve code

vendor/github.com/olivere/elastic/v7/doc.go

@@ -0,0 +1,51 @@
+// Copyright 2012-present Oliver Eilhard. All rights reserved.
+// Use of this source code is governed by a MIT-license.
+// See http://olivere.mit-license.org/license.txt for details.
+
+/*
+Package elastic provides an interface to the Elasticsearch server
+(https://www.elastic.co/products/elasticsearch).
+
+The first thing you do is to create a Client. If you have Elasticsearch
+installed and running with its default settings
+(i.e. available at http://127.0.0.1:9200), all you need to do is:
+
+	client, err := elastic.NewClient()
+	if err != nil {
+		// Handle error
+	}
+
+If your Elasticsearch server is running on a different IP and/or port,
+just provide a URL to NewClient:
+
+  // Create a client and connect to http://192.168.2.10:9201
+  client, err := elastic.NewClient(elastic.SetURL("http://192.168.2.10:9201"))
+  if err != nil {
+    // Handle error
+  }
+
+You can pass many more configuration parameters to NewClient. Review the
+documentation of NewClient for more information.
+
+If no Elasticsearch server is available, services will fail when creating
+a new request and will return ErrNoClient.
+
+A Client provides services. The services usually come with a variety of
+methods to prepare the query and a Do function to execute it against the
+Elasticsearch REST interface and return a response. Here is an example
+of the IndexExists service that checks if a given index already exists.
+
+	exists, err := client.IndexExists("twitter").Do(context.Background())
+	if err != nil {
+		// Handle error
+	}
+	if !exists {
+		// Index does not exist yet.
+	}
+
+Look up the documentation for Client to get an idea of the services provided
+and what kinds of responses you get when executing the Do function of a service.
+Also see the wiki on Github for more details.
+
+*/
+package elastic