Bug 837678 - [cost control] Implement new startup loading events

Per your comments:

 > ..., whether that be by removing a class name, removing `display: none`, etc.

And:

 > This event should be triggered when the app's core interface or primary content are visible.

It they should be visible, then these events are placed properly. Not blocking on this. I was referring to ready to be visible as the comments pointed because is not the same _ready to be visible_ (but invisible, for instance) and _should be visible_.

Thank you @eliperelman !