Add no-referrer policy

Author: rossabaker

_includes/_head.html

@@ -4,7 +4,7 @@
   <title>{{ site.name }} | {% if page.path contains '_event' %}{{ page.short_title }}{% else %}{{ page.title }}{% endif %}</title>
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <meta name="description" content="{{ site.description }}">
-
+  <meta name="referrer" content="no-referrer">
 
   <link rel="mask-icon" href="/img/favicon-safari.svg" color="#F51C2B">
   <link href="/img/favicon.png" rel="icon" type="image/x-icon">