[MM-9937] Add support for sending a message to a different channel than where the slash command was issued from (#9878)

* add support for channel_id in a slash command response

* add test for HandleCommandResponsePost

model/command_response_test.go

@@ -65,6 +65,7 @@ func TestCommandResponseFromJson(t *testing.T) {
 				"response_type": "ephemeral",
 				"text": "response text",
 				"username": "response username",
+				"channel_id": "response channel id",
 				"icon_url": "response icon url",
 				"goto_location": "response goto location",
 				"attachments": [{
@@ -87,6 +88,7 @@ func TestCommandResponseFromJson(t *testing.T) {
 				ResponseType: "ephemeral",
 				Text:         "response text",
 				Username:     "response username",
+				ChannelId:    "response channel id",
 				IconURL:      "response icon url",
 				GotoLocation: "response goto location",
 				Attachments: []*SlackAttachment{