User authentication with sessions

We implemented the functionalities that register new users in our system. The next step is to authenticate these users. Let's first provide an interface to enter a username and password. We need to add a new route handler in frontend/js/app.js:

Router
.add('login', function() {
    var p = new Login();
    showPage(p);
})

All the other pages so far use the same idea. We will initialize a new controller and pass it to the showPage helper. The template that is used here is as follows:

// frontend/tpl/login.html
<header>
  <navigation></navigation>
</header>
<div class="hero">
  <h1>Login</h1>
</div>
<form>
  {{#if error && error != ''}}
    <div class="error">{{error}}</div>
  {{/if}}
  {{#if success && success != ''}}
    <div class="success">{{{success}}}</div>
  {{else}}
    <label for="email">Email</label>
    <input type="text" id="email" value="...