Fix page layout bug

I stumbled upon this bug while working on !15.

Currently the body element is a 100% high flex container with its content vertically centred. This works perfectly when the content is totally contained within the body element.

When the content grows taller than the body element, however, it leads to a situation where part of the content flows out of the top of the viewport and cannot be scrolled to, as demonstrated in this plunk https://plnkr.co/edit/tqAf08kTLTE2miz2?open=lib%2Fscript.js&preview.