return global events from require(['base/js/events']).

In master, it is impossible to register with IPython events in custom.js without setting timeouts because the events doesn't exist yet.

You can now load events in custom.js with:

```javascript
require(["base/js/events"], function (events) {
        
        events.on("notebook_loaded.Notebook", function () {
            console.log("The notebook finished loading!");
        });
});
```
add customjs param to function call, or it will break the notebook on thomas #6126 that also add a requirecall **and** a parameter to the function.