[refs rachelandrew#16] Remove @imported CSS

The @import directive creates a very synchronous, sequential way of
downloading CSS, completely removing our ability to parallelise. By
breaking our CSS out into two separate link elements in the HTML, we can
begin to download the files completely side-by-side.

_includes/head.html

@@ -4,5 +4,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
     <title>{% if page.title %}{{ page.title }}{% else %}{{ site.title }}{% endif %}</title>
     <link rel="canonical" href="{{ page.url | replace:'index.html','' | prepend: site.baseurl | prepend: site.url }}">
-	  <link rel="stylesheet" href="/css/common.css" />
+
+    <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic" />
+    <link rel="stylesheet" href="/css/common.css" />
   </head>

css/common.css

@@ -1,6 +1,5 @@
 @charset "UTF-8";
 /*! Gutenberg reset — adapted from Normalize */
-@import url(https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic);
 html {
   -ms-text-size-adjust: 100%;
   -webkit-text-size-adjust: 100%;