Split fun-fact-box into fun-fact-box and breaking-box

Boxes can be fun-fact-boxes without breaking page flow, and thus the
flow-breaking styles have been moved to the breaking-box class.
Adjust box dimensions and increase size of action buttons when on mobile

On mobile devices, boxes should take the entire width of the screen
but only use as much height as they need. Action buttons should be larger
and more thumb-friendly.