[A11y] switch modal dialogs to <dialog> tag (#5128)

* add templatetag {% dialog %} using <dialog> tag
* new dialog style
* show dialog when empty add-to-cart instead of disabling the button
* update cookieconsent-modal to use new template tag

src/pretix/presale/templates/pretixpresale/event/index.html

@@ -7,6 +7,8 @@
 {% load thumb %}
 {% load eventsignal %}
 {% load rich_text %}
+{% load icon %}
+{% load dialog %}
 
 {% block title %}
     {% if "year" in request.GET %}
@@ -240,6 +242,13 @@
                     </div>
                 {% endif %}
             </form>
+            {% if ev.presale_is_running and display_add_to_cart %}
+                {% trans "You didn’t select any ticket." as label_nothing_to_add %}
+                {% trans "Please tick a checkbox or enter a quantity for one of the ticket types to add to the cart." as description_nothing_to_add %}
+                {% dialog "dialog-nothing-to-add" label_nothing_to_add description_nothing_to_add icon="exclamation-circle" %}
+                    <p class="modal-card-confirm"><button class="btn btn-primary">{% trans "OK" %}</button></p>
+                {% enddialog %}
+            {% endif %}
         {% endif %}
     {% endif %}
     </main>