Allow custom Messenger apps to trigger a larger Messenger view (expand)

  Right now custom apps can’t trigger the same “expand” behavior you see in News and Help
It would be really useful to let a custom app (especially a Sheet app) request a larger Messenger window when a user clicks into it, so the app has more room to display content without feeling cramped.
This would make custom apps feel like first class experiences inside the Messenger, not second class compared to News and Help.