Media queries for the header

I've started on the media queries. Right now, there's just one media
query at a min-width of 800px, but I will very likely add more in the
future. I want to make sure my layout is responsive first and foremost.
I've used floats to position the heading and the navigation right next
to each other. Additionally, I've used flexbox for the navigation
buttons. I will have to make sure this is backwards compatible later on.