Add an empty favicon

There are multiple calls to the favicon on the preview.ideditor.com system in one session:
 > GET http://preview.ideditor.com/favicon.ico 403 (Forbidden)

According to https://stackoverflow.com/questions/1321878/how-to-prevent-favicon-ico-requests#comment50777781_13416784 this should sho an empty favicon which will prevent the browser to try loading the missing icon.

dist/index.html

@@ -4,6 +4,7 @@
         <meta charset='utf-8'>
         <title>iD</title>
         <link rel='stylesheet' href='iD.css'>
+        <link rel="icon" type="image/png" href="data:image/png;base64,iVBORw0KGgo=">
 
         <!-- mobile devices -->
         <meta name='viewport' content='initial-scale=1.0 maximum-scale=1.0'>