See gh-2499

The idea is that custom.js code which was previously listening for app_initialized.NotebookApp events can do this instead:

define([
        'base/js/namespace',
        'base/js/promises'
     ], function(Jupyter, promises) {
         promises.app_initialized.then(function(appname) {
             if (appname === 'NotebookApp') {
                 do_stuff()
             }
         });
     });

I've milestoned this for 5.1 so that we consider it, as I think this is the answer to a long-running annoyance with the event firing before the code listens for it. But I don't mind if we decide to push it back to a later release.