Define a chat responder for the Slack app

This ensures that the Slack application service can be used with
chatops.

ee/app/services/projects/slack_application_install_service.rb

@@ -28,6 +28,10 @@ module Projects
       success
     end
 
+    def chat_responder
+      Gitlab::Chat::Responder::Slack
+    end
+
     private
 
     def make_sure_chat_name_created(slack_data)

ee/changelogs/unreleased/chat-responder-slack-app.yml

+---
+title: Define a chat responder for the Slack app
+merge_request:
+author:
+type: fixed

ee/spec/services/projects/slack_application_install_service_spec.rb

@@ -46,4 +46,12 @@ describe Projects::SlackApplicationInstallService do
     expect_slack_integration_is_created(project)
     expect_chat_name_is_created(project)
   end
+
+  describe '#chat_responder' do
+    it 'returns the chat responder to use' do
+      srv = service
+
+      expect(srv.chat_responder).to eq(Gitlab::Chat::Responder::Slack)
+    end
+  end
 end