[API] add endpoint to check notifications [Extend #9488] (#9595)

* introduce GET /notifications/new

* add TEST

* use Sprintf instead of path.Join

* Error more verbose

* return number of notifications if unreaded exist

* 200 http status for available notifications

routers/api/v1/api.go

@@ -518,6 +518,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 			m.Combo("").
 				Get(notify.ListNotifications).
 				Put(notify.ReadNotifications)
+			m.Get("/new", notify.NewAvailable)
 			m.Combo("/threads/:id").
 				Get(notify.GetThread).
 				Patch(notify.ReadThread)