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

In master, it is impossible to load code on IPython events without setting timeouts.
don’t use events !== undefined to signal page ready

it will always be defined
Trigger app_initialized event *after* the 'global' IPython object is initialized
preserve backward-compatible $([IPython.events])
don’t load custom.js in page template

we are loading it in require, and don’t want to load it twice
fix docstring in events.js

example shows registering a handler, not triggering an event.
note loaded, unused dependencies in require
use global events in js tests
Merge pull request #6119 from minrk/global-events

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