PLT-1649: add response_url support for custom slash commands (#6739)

* add response_url support for custom slash commands

* pr suggestions

* pr update / suggestion

* test fix

api/webhook.go

@@ -32,8 +32,7 @@ func InitWebhook() {
 	BaseRoutes.Hooks.Handle("/{id:[A-Za-z0-9]+}", ApiAppHandler(incomingWebhook)).Methods("POST")
 
 	// Old route. Remove eventually.
-	mr := app.Srv.Router
-	mr.Handle("/hooks/{id:[A-Za-z0-9]+}", ApiAppHandler(incomingWebhook)).Methods("POST")
+	BaseRoutes.Root.Handle("/hooks/{id:[A-Za-z0-9]+}", ApiAppHandler(incomingWebhook)).Methods("POST")
 }
 
 func createIncomingHook(c *Context, w http.ResponseWriter, r *http.Request) {