Add favicon.

layouts/header.html

@@ -7,7 +7,11 @@
     <meta name="description" content="Prometheus monitoring system and time series database">
     <meta name="keywords" content="prometheus, monitoring, monitoring system, time series, time series database, alerting, metrics, telemetry">
     <meta name="author" content="Prometheus">
-    <link rel="icon" href="../../favicon.ico">
+
+    <link rel="shortcut icon" href="/assets/favicon.ico">
+    <link rel="icon" type="image/png" href="/assets/favicon-96x96.png" sizes="96x96">
+    <link rel="icon" type="image/png" href="/assets/favicon-16x16.png" sizes="16x16">
+    <link rel="icon" type="image/png" href="/assets/favicon-32x32.png" sizes="32x32">
 
     <title>Prometheus</title>