Improve cookie auth example

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

PUBLISHED_FROM=eade9dfe283952ed5465cd0ff1924f61a14dea0c

examples/cookie_auth/login.html

@@ -2,7 +2,7 @@
 <html lang="en">
 <head>
   <meta charset="utf-8" />
-  <title>Please log in</title>
+  <title>Log in - Mongoose cookie_auth</title>
   <meta name="viewport" content="width=device-width, initial-scale=1" />
   <style type="text/css">
     * { outline: none; }
@@ -12,7 +12,7 @@
       font: 16px/1.4 Helvetica, Arial, sans-serif;
     }
     div.content {
-      width: 800px; margin: 2em auto; padding: 20px 50px;
+      width: 600px; margin: 2em auto; padding: 20px 50px;
       background-color: #fff; border-radius: 1em;
     }
     label { display: inline-block; min-width: 7em; }