Fix horizontal scroll overflow due to header animation

The header is now moved up and outside of the body container, now allowing the header to take up a max of 100% of the viewport width. With overflow set to hidden, the animation will go outside the viewport but won't create a scrollbar anymore.

_layouts/default.html

@@ -2,8 +2,8 @@
 layout: base
 ---
 
+{% include header.html %}
 <div class="body-container">
-    {% include header.html %}
     {% if page.hide_header or layout.hide_header %}
     {% else %}
     {% if page.title %}