cmd/mbf: bring back pkgserver's favicon!

It existed in mae's #33, but ozy was not satisfied with including
a binary file identical to https://hakurei.app's favicon, and hence
removed it. However, it's possible to explicitly specify the favicon
with a link tag [1]; provided a content security policy that isn't too
strong, this should work fine.

[1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Attributes/rel#icon

cmd/mbf/internal/pkgserver/ui/index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <link rel="stylesheet" href="style.css">
+    <link rel="icon" href="https://hakurei.app/favicon.ico"/>
     <title>Hakurei PkgServer</title>
     <script src="index.js"></script>
 </head>
@@ -54,4 +55,4 @@
 </footer>
 <script>main();</script>
 </body>
-</html>
+</html>