Accept & store thread IDs for receipts (implement MSC3771). (#13782)

Updates the `/receipts` endpoint and receipt EDU handler to parse a
`thread_id` from the body and insert it in the database.

tests/handlers/test_appservice.py

@@ -447,6 +447,7 @@ class ApplicationServicesHandlerSendEventsTestCase(unittest.HomeserverTestCase):
                     receipt_type="m.read",
                     user_id=self.local_user,
                     event_ids=[f"$eventid_{i}"],
+                    thread_id=None,
                     data={},
                 )
             )