[refs rachelandrew#16] Move Twitter widget into head

Seeing as though we’re loading the Twitter widget.js asynchronously, it
doesn’t need to be defined late/JIT. We can move it up to the head to
get it coming over the wire ASAP, but still ensure that it’s not
blocking anything else.

_includes/head.html

@@ -9,4 +9,5 @@
 
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic" />
     <link rel="stylesheet" href="/css/common.css" />
+    <script src="//platform.twitter.com/widgets.js" async defer></script>
   </head>

_includes/sidebar.html

@@ -3,7 +3,7 @@
   <div class="wrapper">
 
     <div class="about">
-      <div class="follow"><a href="https://twitter.com/rachelandrew" class="twitter-follow-button" data-size="large" data-show-screen-name="true" data-show-count="true">Follow @rachelandrew</a><script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script></div>
+      <div class="follow"><a href="https://twitter.com/rachelandrew" class="twitter-follow-button" data-size="large" data-show-screen-name="true" data-show-count="true">Follow @rachelandrew</a></div>
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