Add no-referrer referrer policy to prevent leakage of sensitive info

This prevents Referer headers from being sent when requesting any
external assets, and when clicking on any offsite links, such as the
github link in the top-right corner of the page. This helps to prevent
the leakge of sensitive details, such as private domain names.

assets/templates/layout.html

@@ -5,6 +5,7 @@
     <meta charset="utf-8">
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1">
+    <meta name="referrer" content="no-referrer">
 
     <link rel="icon" type="image/png" href="images/hog.png">