Deprecate bulma in favor of normalize and milligram

Deprecate the bulma css framework in favor of normalize and
milligram. Adapt the templates to work with the new css libraries.

templates/404.html

@@ -1,11 +1,13 @@
 {% extends "base.html" %}
 
+{% block title %}
+404
+{% endblock title %}
+
+{% block subtitle %}
+<h2>404</h2>
+{% endblock subtitle %}
+
 {% block content %}
-<navbar class="navbar level">
-  <div class="level-left">
-    <a href="{{ config.base_url }}">home</a>
-  </div>
-</navbar>
-<h1 class="title">404</h1>
-<h2 class="subtitle">Diese URL führt zu nichts!</h2>
+Diese URL führt zu nichts!
 {% endblock content %}