Add placeholder to insert server generated metatags. Related to #430

index.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>Pleroma</title>
+    <!--server-generated-meta-->
     <link rel="icon" type="image/png" href="/favicon.png">
     <link rel="stylesheet" href="/static/font/css/fontello.css">
     <link rel="stylesheet" href="/static/font/css/animation.css">