auth: Add a logout button

Add a logout button that clears the auth cookies, logging out the user.
It also tries to remove the auth token from the databse, but will ignore
any error during the database operation.

Do some include clean-ups as well.

templates/index.html.tera

@@ -43,7 +43,10 @@
 <div class="top_bar">
     <h1>{{ title }}</h1>
     {% if user.logged_in == true %}
-        <p>Connecté en tant que <b>{{ user.name }}</b></p>
+        <form class="login" id="logout" action="/{{ week_data.number }}/logout" method="POST">
+            Connecté en tant que <b>{{ user.name }}</b>
+            <button form="logout">Déconnecter</button>
+        </form>
     {% else %}
         <form class="login" id="login" action="/{{ week_data.number }}/login" method="POST">
             <label>Pseudo <input form="login" type="text" name="name"/></label>