Add a request argument to the html_head signals

src/pretix/control/context.py

@@ -16,7 +16,7 @@ def contextprocessor(request):
     }
     _html_head = []
     if hasattr(request, 'event'):
-        for receiver, response in html_head.send(request.event):
+        for receiver, response in html_head.send(request.event, request=request):
             _html_head.append(response)
     ctx['html_head'] = "".join(_html_head)