Use flexbox styles only for capable browsers using modernizr classes

As discussed with @xzyfer briefly, I'm going to scope the flexbox-styles to modernizr classes to avoid the broken positioning of the modal on IE10 and IE11.

Using CSS.supports() excludes Safari/iOS Safari which is not an option from my point of view ...

Feedback welcome @joshamos @dannymidnight @andrewk @xzyfer @BeiYang123