Let plugins allow/prevent the download of individual tickets in an order (#3836)

(extends the functionality of the allow_ticket_download signal)

src/tests/testdummy/ticketoutput.py

@@ -33,3 +33,7 @@ class DummyTicketOutput(BaseTicketOutput):
 
     def generate(self, op):
         return 'test.txt', 'text/plain', str(op.order.id)
+
+    @property
+    def multi_download_enabled(self) -> bool:
+        return False