flexbox tiles

Same height columns have traditionally been complicated (or even impossible) with CSS. The only option were tables which -- apart from other issues -- don't allow absolute positioning of their contents. For us this is an issue with the button position in tiles (#687).

[flexbox](http://www.w3.org/TR/css3-flexbox/) to the rescue. Flexbox is very new CSS. But with the updated modernizr (#747) we can detect it and keep the table layout as a fallback.