[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/swagger/notify.go

@@ -21,3 +21,10 @@ type swaggerNotificationThreadList struct {
 	// in:body
 	Body []api.NotificationThread `json:"body"`
 }
+
+// Number of unread notifications
+// swagger:response NotificationCount
+type swaggerNotificationCount struct {
+	// in:body
+	Body api.NotificationCount `json:"body"`
+}