add extend button to user interface

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

@@ -492,7 +492,10 @@
 <div class="row">
     <div class="col-md-12">
     {% if not cart.is_ordered %}
-        <p class="text-muted" id="cart-deadline" data-expires="{{ cart.first_expiry|date:"Y-m-d H:i:sO" }}">
+        <form class="text-muted"
+                method="post" data-asynctask action="{% eventurl request.event "presale:event.cart.extend" cart_namespace=cart_namespace %}">
+            {% csrf_token %}
+            <span id="cart-deadline" data-expires="{{ cart.first_expiry|date:"Y-m-d H:i:sO" }}">
                 {% if cart.minutes_left > 0 or cart.seconds_left > 0 %}
                     {% blocktrans trimmed with minutes=cart.minutes_left %}
                         The items in your cart are reserved for you for {{ minutes }} minutes.
@@ -500,7 +503,10 @@
                 {% else %}
                     {% trans "The items in your cart are no longer reserved for you. You can still complete your order as long as they’re available." %}
                 {% endif %}
-        </p>
+            </span>
+            <button class="btn btn-link" type="submit" id="cart-extend-button">
+                <i class="fa fa-refresh" aria-hidden="true"></i> {% trans "Extend" %}</button>
+        </form>
     {% else %}
         <p class="sr-only" id="cart-description">{% trans "Overview of your ordered products." %}</p>
     {% endif %}

src/pretix/static/pretixpresale/js/ui/cart.js

@@ -55,6 +55,7 @@ var cart = {
                 pad(diff_minutes.toString(), 2) + ':' + pad(diff_seconds.toString(), 2)
             );
         }
+        $("#cart-extend-button").toggle(diff_minutes < 3);
     },
 
     init: function () {