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/CHANGELOG-7.0.md

@@ -0,0 +1,55 @@
+# Changes from 6.0 to 7.0
+
+See [breaking changes](https://www.elastic.co/guide/en/elasticsearch/reference/7.x/breaking-changes-7.0.html).
+
+## SearchHit.Source changed from `*json.RawMessage` to `json.RawMessage`
+
+The `SearchHit` structure changed from
+
+```
+// SearchHit is a single hit.
+type SearchHit struct {
+	...
+	Source         *json.RawMessage               `json:"_source,omitempty"`         // stored document source
+	...
+}
+```
+
+to
+
+```
+// SearchHit is a single hit.
+type SearchHit struct {
+	...
+	Source         json.RawMessage                `json:"_source,omitempty"`         // stored document source
+	...
+}
+```
+
+As `json.RawMessage` is a `[]byte`, there is no need to specify it
+as `*json.RawMessage` as `json.RawMessage` is perfectly ok to represent
+a `nil` value.
+
+So when deserializing the search hits, you need to change your code from:
+
+```
+for _, hit := range searchResult.Hits.Hits {
+    var doc Doc
+    err := json.Unmarshal(*hit.Source, &doc) // notice the * here
+    if err != nil {
+        // Deserialization failed
+    }
+}
+```
+
+to
+
+```
+for _, hit := range searchResult.Hits.Hits {
+    var doc Doc
+    err := json.Unmarshal(hit.Source, &doc) // it's missing here
+    if err != nil {
+        // Deserialization failed
+    }
+}
+```