Allow to pass user data to the widget (#1095)

- [x] Logic
- [x] Tests
- [x] Docs
- [x] find a way to integrate with tracking

src/pretix/presale/views/event.py

@@ -189,6 +189,9 @@ class EventIndex(EventViewMixin, CartMixin, TemplateView):
             return redirect(eventreverse(request.event, 'presale:event.index', kwargs=kwargs) + '?require_cookie=true&cart_id={}'.format(
                 request.GET.get('take_cart_id')
             ))
+        elif request.GET.get('iframe', '') == '1' and len(self.request.GET.get('widget_data', '{}')) > 3:
+            # We've been passed data from a widget, we need to create a cart session to store it.
+            get_or_create_cart_id(request)
         elif 'require_cookie' in request.GET and settings.SESSION_COOKIE_NAME not in request.COOKIES:
             # Cookies are in fact not supported
             r = render(request, 'pretixpresale/event/cookies.html', {