Sidebar should disappear on small screens.

We've had a report that the sidebars are very distracting,
particularly for the blogs, on smaller screens.  This change to the
CSS will cause sidebars to disappear on smaller screens.

www/conservancy/static/css/conservancy.css

@@ -151,6 +151,12 @@ text-decoration: none; color: #557733;
   }
 }
 
+@media screen and (max-width: 52em) { 
+  #sidebar { 
+      display: none; 
+  } 
+} 
+
 p.footnote {
     font-size: 85%;
 }