SSP-1521 Fix stalled navigation on completing caseload assignment

The event handler that dealt with this was only registered if a
student had already been selected. Also seemed to be some sort of
race where it could still occasionally be registered even if no
student were selected.

The reason it worked this way, rather than registering the listener
at component initialization, was that the listening component
(MainViewController.js) was created by the default view
(Viewport), which was being created before the Deft IoC container
had been fully initialized. Specifically, the Ext Application
had not yet been constructed so had not yet been injected into
AppEventsController. So MainViewController had to rely
on view- rather than Application-scoped events since the former
apparently didn't fire until the AppEventsController had finally
been completely initialized. These view-scoped events were clearly
unreliable for this purpose, though, and could result in the
App-scoped listener never being registered.

The fix was to delay initialization of Viewport until after the
Application had been fully initialized.