Add mobile support in the slide app

www/slide/index.html

@@ -3,6 +3,7 @@
 <head>
     <title>CryptPad</title>
     <meta content="text/html; charset=utf-8" http-equiv="content-type"/>
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <script data-bootload="main.js" data-main="/common/boot.js" src="/bower_components/requirejs/require.js"></script>
     <link rel="icon" type="image/png"
         href="/customize/main-favicon.png"