Improve cookie auth example

Use templated index page and SSI calls instead of hard-coded html

PUBLISHED_FROM=eade9dfe283952ed5465cd0ff1924f61a14dea0c

examples/cookie_auth/index.shtml

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+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="utf-8" />
+  <title>Main - Mongoose cookie_auth</title>
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+  <style type="text/css">
+    * { outline: none; }
+    body {
+      background-color: #789; margin: 0;
+      padding: 0; font: 16px/1.4 Helvetica, Arial, sans-serif;
+      font: 16px/1.4 Helvetica, Arial, sans-serif;
+    }
+    div.content {
+      width: 600px; margin: 2em auto; padding: 20px 50px;
+      background-color: #fff; border-radius: 1em;
+    }
+    label { display: inline-block; min-width: 7em; }
+    input { border: 1px solid #ccc; padding: 0.2em; }
+    a:link, a:visited { color: #69c; text-decoration: none; }
+    @media (max-width: 700px) {
+      body { background-color: #fff; }
+      div.content { width: auto; margin: 0 auto; padding: 1em; }
+    }
+</style>
+</head>
+<body>
+  <div class="content">
+    <h1>Hello, <!--#call user -->!</h1>
+    <p>Your lucky number is <!--#call lucky_number -->.</p>
+    <p><a href="/logout">Log out</a>
+  </div>
+</body>
+</html>