style: make registration flow use cards

src/templates/django_registration/activation_form.html

@@ -3,13 +3,15 @@
 {% load crispy_forms_tags %}
 
 {% block content %}
+    <div class="card">
     {% if not user.is_authenticated %}
         <form action="" method="post">
             {% csrf_token %}
             {{ form.as_p }}
-            <input type="submit" class="btn2" value={% translate 'Absenden' %}>
+            <input type="submit" class="btn" value={% translate 'Absenden' %}>
         </form>
     {% else %}
         <p>{% translate "Du bist bereits eingeloggt." %}</p>
     {% endif %}
+    </div>
 {% endblock %}