Prevent switching of preferred supervent once chosen in raven.py

There is at least one instance of the preferred superevent trying to chang well after the alerts are sent out:

https://gracedb-playground.ligo.org/superevents/S230407bl/view/ https://ligo-caltech.sentry.io/issues/2956938559/events/7354860c5a0e44498658fe1e61301d33/?project=1425216

Currently external events associated with superevents are not able to be switched without being removed, so we should prevent the switching the first place. This will require adding a condition here that ensure the superevent being looked at is the same already associated with the external event or is None (i.e. the external event has no association yet), ignoring calculating data products/switching for later associations with superevents: https://git.ligo.org/emfollow/gwcelery/-/blob/99dae3747de44c2e9465c140bb95ef89a9376ce7/gwcelery/tasks/raven.py#L220

This could be done by adding some logic within the current else statement block:

else:
    ...
    # Don't continue if different superevent than previous
    if ext_event['superevent'] is not None and ext_event['superevent'] != superevent['superevent_id']:
        return