Rename the favicon to avoid 404 errors

app/view/header.html

@@ -3,7 +3,7 @@
     <meta charset="utf-8" />
     <meta name="author" content="La Quadrature du Net" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <link rel="icon" type="image/png" href="favicon.png" />
+    <link rel="icon" type="image/png" href="favicon.ico" />
 
     <!-- Open graph -->
     <meta property="og:title" content="{{ _('Soutenez La Quadrature du Net !') }}" />