Fix incorrect key when querying external events

The key 'superevent_id' correctly returns the superevent gracedb ID when querying a superevent, while 'superevent' is the correct key for external events (see gracedb documentation). However, 'superevent_id' was being called for both which is incorrect, and if ever occurred, would have lead to a KeyError. This merge request now calls the correct key when querying external events.

It's honestly surprising this serious bug hasn't been found, but this is due to the fact that we only query external events when the creation of a superevent triggers RAVEN (i.e. external event before superevent).