add a cancel button to the WarrantyAddView

devices/templates/devices/warranty_create.html

@@ -8,6 +8,7 @@
     {% csrf_token %}
     {{ form|crispy }}
     <button type="submit" class="btn btn-primary">Save</button>
+    <a href="{{ view.get_success_url }}" class="btn btn-secondary">Cancel</a>
 </form>
 </div>
 {% endblock %}