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.

synapse/rest/client/read_marker.py

@@ -83,6 +83,8 @@ class ReadMarkerRestServlet(RestServlet):
                     receipt_type,
                     user_id=requester.user.to_string(),
                     event_id=event_id,
+                    # Setting the thread ID is not possible with the /read_markers endpoint.
+                    thread_id=None,
                 )
 
         return 200, {}