Laying out the directory grid using flexbox

@moneyadviceservice/frontend @amansinghb 

I have experimented with using flex box to layout the home page grid. It doesn't need a min-height and stacks nicely. I have wrapped the styles in a responsive-switch so IE8 and IE9 get custom fallback styles.

Would be interested to hear your thoughts on whether we could use more of this for laying out internal site elements (as suggested by Andy Hume in workshop). 
Looks like it should be a known property
https://github.com/CSSLint/parser-lib/blob/master/src/css/Properties.js